Rubico Inc美股招股说明书(2026-04-21版)

PROSPECTUS SUPPLEMENT NO. 16(TO PROSPECTUS DATED SEPTEMBER 19, 2025) Up to 15,000,000 Common Shares RUBICO INC. This is a supplement (the “Prospectus Supplement”) to the prospectus, dated September 19, 2025 (as supplemented oramended from time to time, the “Prospectus”) of Rubico Inc. (the “Company”), which forms a part of the Company’s RegistrationStatement on Form F-1 (Registration Nos. 333-288796, 333-291884, 333-293441 and 333-294944), as amended from time to time. This Prospectus Supplement is being filed to update and supplement the information included in the Prospectus with theinformation contained in the Company’s Report on Form 6-K, furnished to the U.S. Securities and Exchange Commission (the“Commission”) on April 21, 2026 (the “Form 6-K”). Accordingly, the Form 6-K is attached to this Prospectus Supplement. This Prospectus Supplement should be read in conjunction with, and delivered with, the Prospectus and is qualified byreference to the Prospectus except to the extent that the information in this Prospectus Supplement supersedes the informationcontained in the Prospectus. This Prospectus Supplement is not complete without, and may not be delivered or utilized except in connection with, theProspectus, including any amendments or supplements to it. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 12 of the Prospectusfor a discussion of information that should be considered in connection with an investment in our securities. Neither the Commission nor any state securities commission has approved or disapproved of these securities ordetermined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense. The date of this prospectus supplement is April 21, 2026. UNITED STATES SECURITIES AND EXCHANGE COMMISSIONWashington, D.C. 20549 FORM 6-K REPORT OF FOREIGN PRIVATE ISSUER PURSUANT TO RULE 13A-16 OR 15D-16 UNDER THE SECURITIESEXCHANGE ACT OF 1934 For the month of April 2026 Commission File Number: 001-42684 (Translation of registrant’s name into English) 20 Iouliou Kaisara Str19002 PaianiaAthens, 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 REPORTCommon Share Purchase Agreement On April 20, 2026, Rubico Inc. (the “Company”) entered into a Common Share Purchase Agreement (the “PurchaseAgreement”) with B. Riley Principal Capital II, LLC (“B. Riley Principal Capital II”). Pursuant to the Purchase Agreement, theCompany has the right to sell to B. Riley Principal Capital II up to $50.0 million of its common shares, $0.01 par value per share(the “Common Shares”), subject to certain limitations and conditions set forth in the Purchase Agreement, from time to time duringthe term of the Purchase Agreement. Sales of Common Shares pursuant to the Purchase Agreement, and the timing of any sales, aresolely at the option of the Company, and the Company is under no obligation to sell any securities to B. Riley Principal Capital IIunder the Purchase Agreement. Upon the initial satisfaction of the conditions to B. Riley Principal Capital II’s purchase obligations set forth in the PurchaseAgreement (the “Commencement”), including that the Initial Registration Statement (as defined below), required to be filed by theCompany pursuant to the Registration Rights Agreement (as defined below) described under “Registration Rights Agreement”below, has been declared effective by the U.S. Securities and Exchange Commission (the “SEC”), the Company will have the right,but not the obligation, from time to time at its sole discretion over the 36-month period beginning on the date the Commencementoccurs (the “Commencement Date”), to direct B. Riley Principal Capital II to purchase a specified number of the Company’sCommon Shares, not to exceed certain limitations as set forth in the Purchase Agreement (each, a “Purchase”). Each Purchase shallnot exceed the lesser of 1,000,000 of the Common Shares and a percentage to be specified by the Company, not to exceed 25%,times the aggregate number of the Common Shares traded on the Nasdaq Capital Market (“Nasdaq”) during the applicablePurchase Valuation Period (as defined below). In addition, the Company may only initiate a Purchase if the closing sale price of theCommon Shares on Nasdaq on the preceding trading day is not less than $1.00. Any such direction will be made by the Companytimely delivering written notice to B. Riley Principal Capital II (each, a “Purchase Notice”) prior to 9:00 a.m., New York City time,on any trading day (each, a “Purchase Date”), subject to certain requirements as set forth in the Purchase Agreement. From and after Commencement, the Company will control the timing and amount of any sales of its Common Shares to B.Riley Principal Capital II. Actual sales of the Company’s Com