46 C.F.R. § 4.03-40

Public vessel means a vessel that—

(a) Is owned, or demise chartered, and operated by the U.S. Government or a government of a foreign country, except a vessel owned or operated by the Department of Transportation or any corporation organized or controlled by the Department (except a vessel operated by the Coast Guard or Saint Lawrence Seaway Development Corporation); and

(b) Is not engaged in commercial service.