A vessel must measure at least five net tons and must be wholly owned by a citizen of the United States. There are certain exceptions that apply. Usually a vessel 25' in length can qualify for documentation. The tonnage is determined by volume of the vessel, not by how much the manufacturer says the vessel weights. Vessels that engage in certain trades like coastwise, fishery and registry are required by law to be federally documented.
Tonnage is determined by volume.
Most vessels more than 25 feet in length will measure five net tons or more. Certain other rules apply. A Certificate of Documentation may be endorsed for fishery, coastwise, registry, or recreation. Any documented vessel may be used for recreational purposes, regardless of its endorsement, but a vessel documented with a recreational endorsement only may not be used for any other purpose. Registry endorsements are generally used for foreign trade.