29 C.F.R. § 783.41
A vessel "numbered under the laws of the United States" means a vessel numbered pursuant to the provisions of Federal law, including vessels numbered under any State numbering system approved by the Secretary of the Department under which the U.S. Coast Guard is operating, in accordance with section 2(c) of the Federal Boating Act of 1958 (46 U.S.C. 527-527h ). Generally, any vessel, which is not required to have and does not have, a valid marine document issued by the Bureau of Customs and is propelled by machinery of more than 10 horsepower, whether or not such machinery is the principal source of propulsion, is required to be numbered in conformity with the Federal Boating Act of 1958 if it uses the navigable waters of the United States, its Territories, or the District of Columbia, or is owned in a State and uses the high seas (46 U.S.C. 527(a) ). The requirements and procedures of this Act are explained in detail in 46 CFR part 170.
29 C.F.R. §783.41