To qualify for USCG documentation, a vessel must be wholly owned by a citizen of the United States and measure at least five net tons. Net tonnage is a measurement not of weight but of volume. A simple rule: if a vessel measures more than 25 feet in length, it almost invariably measures at least five net tons.
Vessels that meet the above requirements can be documented. Some vessels must be documented. These include vessels that are used for transporting people and/or goods on the navigable waters of the United States or in the EEZ (Exclusive Economic Zone). Additionally, vessels that engage in commercial fishing activities on those same waters must be documented as well.
Beyond domestic trade, vessels that engage in foreign trade may be eligible for the “Registry” endorsement.
Apply for vessel documentation here.
Section 67.5 goes into detail on requirements.