41 C.F.R. § 102-117.50

(a) Coastwise laws (46 U.S.C. chapter 551) require the use of U.S.-flag, coastwise-endorsed vessels for domestic water shipments between U.S. ports.

(b) For exceptions and further information, refer to regulations issued by the U.S. Customs and Border Protection (CBP) (19 CFR 4.80) and MARAD.