UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER PURSUANT TO RULE 13A-16 OR 15D-16 UNDER THE SECURITIES EXCHANGE ACT OF 1934

For the month of November 2025

Commission File Number: 001-42684

RUBICO INC.

(Translation of registrant’s name into English)

20 Iouliou Kaisara Str
19002 Paiania

Athens, Greece

(Address of principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

Form 20-F ☒ Form 40-F ☐

INFORMATION CONTAINED IN THIS FORM 6-K REPORT

Public Offering

On November 6, 2025, Rubico Inc. (“we” or the “Company”), closed a firm commitment underwritten public offering (the “Offering”) of 12,315,270 units (“Units”), each Unit consisting of one share of our common stock, par value $0.01 per share (“Common Shares”) and one Class A Warrant (each, a “Warrant”) to purchase one Common Share, at a public offering price of $0.609 per Unit.

The Company has also granted Maxim Group LLC, the representative of the underwriters in the Offering (the “Representative”), a 45-day option to purchase up to an additional 1,847,290 additional Common Shares at the offering price and/or up to 1,847,290 Warrants, to cover over-allotments, if any. On November 6, 2025, the Representative partially exercised its over-allotment option with respect to 1,847,290 Warrants.

In addition, we agreed to issue to the Representative warrants to purchase a number of Common Shares equal to 5.0% of the total number of Common Shares sold in the Offering, including any Common Shares sold pursuant to the over-allotment option granted to the Representative (the “Representative Warrants”). Representative Warrants to purchase 615,763 Common Shares were issued in connection with the closing of the Offering. The Representative Warrants have substantially similar terms as the Warrants issued in the Offering.

The Offering was made pursuant to a registration statement on Form F-1, as amended (File No. 333-290426), filed with the Securities and Exchange Commission (the “Commission”), which became effective with the Commission on November 4, 2025.

The aggregate gross proceeds to the Company from the Offering, before deducting underwriting discounts and other expenses payable by the Company, were approximately $7.5 million.

On November 5, 2025, the Company released a press release furnished herewith as Exhibit 99.1, announcing the pricing of the Offering.

Description of the Warrants

The following summary of certain terms and provisions of the Warrants is not complete and is subject to, and qualified in its entirety by, the provisions of the form of Warrant furnished as an exhibit to this report.

Exercisability. The Warrants are immediately exercisable at any time after their issuance and at any time up to the date that is one (1) year after their issuance. The Warrants will be exercisable, at the option of each holder, in whole or in part by delivering to us a duly executed exercise notice with payment in full in immediately available funds for the number of Common Shares purchased upon such exercise (except in the case of a cashless exercise or zero cash exercise, as discussed below). We may be required to pay certain amounts as liquidated damages as specified in the warrants in the event we do not deliver Common Shares upon exercise of the warrants within the time periods specified in the warrants. No fractional Common Shares will be issued in connection with the exercise of a warrant.

Cashless Exercise and Zero Cash Exercise. If a registration statement registering the issuance of the Common Shares underlying the Warrants under the Securities Act is not effective or available, the holder may, in its sole discretion, elect to exercise the Warrant through a cashless exercise, in which case the holder would receive upon such exercise the net number of Common Shares determined according to the formula set forth in the Warrant.

A holder of Warrants may also provide notice and elect to exercise the Warrant by way of a zero cash exercise option pursuant to which they would receive, for no additional cash consideration, an aggregate number of shares equal to the product of (x) the aggregate number of Common Shares that would be issuable upon a cash exercise of the Warrant and (y) 2.0.

Exercise Limitation. A holder will not have the right to exercise any portion of the Warrants if the holder (together with its affiliates) would beneficially own in excess of 4.99% (or, upon election of the holder prior to the issuance of any Warrants, up to 9.99%) of the number of shares of our Common Shares outstanding immediately after giving effect to the exercise, as such percentage ownership is determined in accordance with the terms of the warrants. However, any holder may increase or decrease such percentage to any other percentage not in excess of 9.99%, upon at least 61 days’ prior notice from the holder to us with respect to any increase in such percentage.

Exercise Price. Each Warrant sold in the Offering will have an initial exercise price per Common Share equal to $0.609. The exercise price for the Warrants will be adjusted downward (each, a “Reset Price”) on each of the 4th and 8th trading day (each, a “Reset Date”) following the closing of the Offering to the price that is equal to $0.4263 and $0.3045, respectively. If the exercise price is so adjusted on a Reset Date, the number of Common Shares underlying the Warrants will also be proportionally increased on each such Reset Date so that the applicable Reset Price multiplied by the increased number of Common Shares equals the aggregate proceeds that would have resulted from the full exercise of the Warrants immediately prior to the applicable Reset Date. The exercise price and number of Common Shares issuable upon exercise are subject to appropriate adjustment in the event of stock dividends, stock splits, reorganizations or similar events affecting our Common Shares.

Transferability. Subject to applicable laws, the Warrants may be offered for sale, sold, transferred or assigned without our consent.

Exchange Listing. We do not intend to list the Warrants on any securities exchange or other trading market. Without an active trading market, the liquidity of these securities will be limited.

Rights as a Shareholder. Except as otherwise provided in the Warrants or by virtue of such holder’s ownership of our Common Shares, the holder of a Warrant does not have the rights or privileges of a holder of our Common Shares, including any voting rights, until the issuance of Common Shares upon exercise of the Warrant. Holders of Warrants have the right to participate in certain distributions as specified in the warrant.

Fundamental Transactions. In the event of a fundamental transaction, as described in the Warrants and generally including, with certain exceptions, any reorganization, recapitalization or reclassification of our Common Shares, the sale, transfer or other disposition of all or substantially all of our properties or assets, our consolidation or merger with or into another person, the acquisition of more than 50% of our outstanding Common Shares or 50% of the voting power represented by our outstanding Common Shares, the holders of the Warrants will be entitled to receive upon exercise of the warrants the kind and amount of securities, cash or other property that the holders would have received had they exercised the Warrants immediately prior to such fundamental transaction.

Governing Law. The Warrants are governed by New York law.

Update on Issued and Outstanding Common Shares

On November 6, 2025, following the closing of the Offering and partial exercise of the Representative’s overallotment option as described above, 17,612,636 Common Shares were issued and outstanding.

In addition, Warrants to purchase 14,162,560 Common Shares and Representative Warrants to purchase 615,763 Common Shares were issued in connection with the closing of the Offering and partial exercise of the Representative’s overallotment option as described above. If all such outstanding Warrants and Representative Warrants are exercised on a zero cash basis following the second Reset Date as described above, a maximum of 59,113,292 shares could be issued upon such zero cash exercise, without payment to the Company of any additional cash.

Supplemental Risk Factors

The risk factors set forth below supplement and should be read together with those risk factors set forth under the heading “Risk Factors” in our registration statement on Form 20-F, filed with the Commission on June 4, 2025.

The market price of our Common Shares has been and may in the future be subject to significant fluctuations.

The market price of our Common Shares has been and may in the future be subject to significant fluctuations as a result of many factors, some of which are beyond our control. Among the factors that could in the future affect our stock price are:

The stock markets in general, and the markets for shipping and shipping stocks in particular, have experienced extreme volatility that has sometimes been unrelated to the operating performance of individual companies. These broad market fluctuations may adversely affect the trading price of our Common Shares.

We may experience rapid and substantial share price volatility unrelated to our actual or expected operating performance, financial condition or prospects, making it difficult for prospective investors to assess the rapidly changing value of our Common Shares.

As a relatively small-capitalization company with relatively small public float, we may experience greater share price volatility, extreme price run-ups or rapid price declines, larger spreads in bid and ask prices, lower trading volume and less liquidity than large-capitalization companies. Such volatility, including any share price run-up, may be unrelated to our actual or expected operating performance, financial condition or prospects, making it difficult for prospective investors to assess the rapidly changing value of our Common Shares. In addition, holders of our Common Shares may experience losses, which may be material, if the price of our Common Shares declines after the Offering or if such investors purchase our Common Shares prior to any price decline.

For example, on November 4, 2025, the trading price of our common shares ranged from an intra-day high of $1.92 to an intra-day low of $0.542, on trading volume of approximately 41.9 million shares.

Furthermore, if the trading volumes of our Common Shares are low, investors buying or selling in relatively small quantities may be able to easily influence the price of our Common Shares. Such low volume of trades could also cause the price of our Common Shares to fluctuate greatly, with large percentage changes in share price occurring in any trading day session. Holders of our Common Shares may also not be able to readily liquidate their investment or may be forced to sell at depressed prices due to low volume trading. Broad market fluctuations and general economic and political conditions may also adversely affect the market price of our Common Shares. As a result of this volatility, investors may experience losses on their investment in our Common Shares. A decline in the market price of our Common Shares also could adversely affect our ability to issue additional Common Shares or other securities and our ability to obtain additional financing in the future. Please also see “Supplemental Risk Factors—The market price of our Common Shares may in the future be subject to significant fluctuations” above.

A possible “short squeeze” due to a sudden increase in demand of our Common Shares that largely exceeds supply may lead to further price volatility in our Common Shares.

Investors may purchase our Common Shares to hedge existing exposure in our Common Shares or to speculate on the price of our Common Shares. Speculation on the price of our Common Shares may involve long and short exposures. To the extent aggregate short exposure exceeds the number of Common Shares available for purchase in the open market, investors with short exposure may have to pay a premium to repurchase our Common Shares for delivery to lenders of our Common Shares. Those repurchases may in turn, dramatically increase the price of our Common Shares until investors with short exposure are able to purchase additional Common Shares to cover their short position. This is often referred to as a “short squeeze.” Following such a short squeeze, once investors purchase the shares necessary to cover their short position, the price of our Common Shares may rapidly decline. A short squeeze could lead to volatile price movements in our Common Shares that are not directly correlated to the performance or prospects of our company.

Nasdaq may halt trading in our Common Shares on Nasdaq or delist our Common Shares for public interest concerns as a result of the Offering.

Because of the highly dilutive nature of the Offering, Nasdaq may halt trading in our Common Shares on Nasdaq or delist our Common Shares for public interest concerns or because our Common Shares continue to trade below Nasdaq’s minimum bid price as a result of the Offering, even if we are otherwise able to regain compliance for continued listing on Nasdaq. A number of Nasdaq-listed companies have filed public disclosures regarding the receipt of notification letters indicating that Nasdaq made the determination to halt and/or delist such companies as a result of public interest concerns arising from the issuance of warrants with similar terms to, and similar potential dilutive impact as, the Warrants in the Offering. Additionally, warrants with similar terms issued by other Nasdaq-listed companies have caused such Nasdaq-listed companies’ stock prices to drop below Nasdaq’s minimum bid price or made it more difficult for these companies to cause their stock prices to regain compliance with Nasdaq’s minimum bid price. As the Offering closed at a price below Nasdaq’s minimum bid price, there can be no assurance that our Common Shares will trade above Nasdaq’s minimum bid price in the future, which may cause Nasdaq to delist our Common Shares.

If Nasdaq delists our securities from trading on its exchange for failure to meet its listing standards, and we are not able to list such securities on another national securities exchange, then our Common Shares could be quoted on an over-the-counter market. If this were to occur, we and our stockholders could face significant material adverse consequences, including:

As a result of the price resets and subsequent share adjustments in the Warrants and/or if the Warrants are exercised by way of a zero cash exercise, common shareholders are likely to suffer substantial dilution and see a significant decrease in the value of their Common Shares.

The Warrants include exercise price reset provisions and a zero cash exercise option, both of which will result in the issuance of a significantly higher number of shares upon exercise. Specifically, the Warrants will be immediately exercisable and may be exercisable until the one (1) year anniversary of the date of issuance at an exercise price equal to $0.609. In addition, subject to certain conditions, the exercise price for the Warrants will be adjusted downward on each of the 4th and 8th trading day following the closing of the Offering to the price that is equal to 70%, and 50%, respectively, of the initial exercise price, and the number of Common Shares underlying the Warrants will be proportionally increased, as further described above. The Warrants also contain certain mechanisms for cashless exercise, including a zero cash exercise option pursuant to which holders of the Warrants have the option, upon exercise and for no additional cash consideration, to receive an aggregate number of Common Shares equal to the product of (x) the aggregate number of Common Shares that would be issuable upon a cash exercise of the Warrant and (y) 2.0.

As an example, for each Unit that an investor purchases in the Offering at the offering price of $0.609 per Unit, the investor will receive 1 Common Share and 1 Warrant to purchase 1 Common Share at an exercise price of $0.609 per Common Share. Giving effect solely to the adjustment on the first Reset Date and not giving effect to the zero cash exercise option, on the first Reset Date, each Warrant will become exercisable for a maximum of 1.4286 Common Shares at an exercise price of $0.4263 per Common Share. If such Warrant is then exercised at such time based on the zero cash exercise option, the Warrant would be exercisable into 2.8571 Common Shares. If the Warrant remains outstanding as of the second Reset Date, giving effect solely to the potential adjustment on the second Reset Date and not giving effect to the zero cash exercise option, on the second Reset Date, each Warrant will become exercisable for a maximum of 2 Common Shares at an exercise price of $0.3045 per Common Share. If such Warrant is then exercised at such time based on the zero cash exercise option, the Warrant would be exercisable into 4 Common Shares. Accordingly, if all of the Warrants offered to investors in the Offering are exercised on a zero cash basis following the second Reset Date, a maximum of 49,261,080 shares could be issued upon such zero cash exercise, without payment to the Company of any additional cash. In addition, we have granted the Representative an option exercisable within 45 days of the pricing of the Offering to purchase from us up to 1,847,290 additional Common Shares and/or up to 1,847,290 Warrants to cover over-allotments, if any. If the Representative’s over-allotment option for Warrants is exercised in full to purchase 1,847,290 Warrants and all such Warrants are exercised on a zero cash basis following the second Reset Date, a maximum of 7,389,160 shares could be issued upon such zero cash exercise without payment to the Company of any additional cash. If all of the Warrants offered to investors in the Offering, including those subject to the Representative’s over-allotment option for Warrants, are exercised on a zero cash basis following the second Reset Date, a maximum of 56,650,240 shares could be issued upon such zero cash exercise without payment to the Company of any additional cash. In addition, the Representative Warrants have substantially similar terms to those of the Warrants including zero cash exercise, and if all the Representative Warrants issued in connection with the Offering (assuming no exercise of the Representative’s over-allotment option) were exercised on a zero cash basis following the second Reset Date, a maximum of an additional 2,463,052 shares (or 2,832,512 shares assuming full exercise of the Representative’s over-allotment option) could be issued upon such zero cash exercise without payment to the Company of any additional cash.

As a result of the terms of the Warrant, it is likely that our stock price will be reduced considerably because of each of the resets. Accordingly, common shareholders are likely to suffer substantial dilution and see a significant decrease in the value of their Common Shares as a result of this transaction.

Cautionary Note Regarding Forward-Looking Statements

Matters discussed in this Report may constitute forward-looking statements. The Private Securities Litigation Reform Act of 1995 provides safe harbor protections for forward-looking statements in order to encourage companies to provide prospective information about their business. Forward-looking statements include statements concerning plans, objectives, goals, strategies, future events or performance, and underlying assumptions and other statements, which are other than statements of historical facts, including statements regarding the Offering.

The Company desires to take advantage of the safe harbor provisions of the Private Securities Litigation Reform Act of 1995 and is including this cautionary statement in connection with this safe harbor legislation. The words “believe,” “anticipate,” “intends,” “estimate,” “forecast,” “project,” “plan,” “potential,” “may,” “should,” “expect,” “pending,” and similar expressions identify forward-looking statements.

The forward-looking statements in this report are based upon various assumptions, many of which are based, in turn, upon further assumptions, including, without limitation, management's examination of historical operating trends, data contained in the Company’s records, and other data available from third parties. Although the Company believes that these assumptions were reasonable when made, because these assumptions are inherently subject to significant uncertainties and contingencies which are difficult or impossible to predict and are beyond the Company’s control, the Company cannot assure you that it will achieve or accomplish these expectations, beliefs, or projections.

In addition to these assumptions, important factors that, in the Company’s view, could cause actual results to differ materially from those discussed in the forward-looking statements include, among other things:

Should one or more of the foregoing risks or uncertainties materialize, or should any of the Company’s assumptions prove incorrect, actual results may vary in material respects from those projected in these forward-looking statements. Consequently, there can be no assurance that actual results or developments anticipated by the Company will be realized or, even if substantially realized, that they will have the expected consequences to, or effects, on it. Given these uncertainties, you are cautioned not to place undue reliance on such forward-looking statements.

The Company undertakes no obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise, except as may be required under applicable laws. If one or more forward-looking statements are updated, no inference should be drawn that additional updates will be made with respect to those or other forward-looking statements.

Exhibit Index

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.