Any structure made to float upon the water for the purpose of commerce or war, whether impelled by wind, steam or oars -this is the explanation of a vessel given in the American-English Encyclopedia of Law. A vessel has been further defined in the Revised Statutes of the United States as including every description of watercraft or other artificial contrivance used, or capable of being used, as a means of transportation by water. A controversial question however, that often creates confusion, is this: When, during the course of construction, is a craft considered to be a full-fledged vessel. Theories and applications of maritime laws of foreign countries, on this as on other points, are often deprecated by the courts of the United States.
Donald W. Peters, What Is a Vessel in Admiralty Law, 6 Clev.-Marshall L. Rev. 139 (1957)