In a personal injury context, under the Supreme Court ruling in Stewart v. Dutra Construction, a vessel includes every description of watercraft or other artificial contrivance used, or capable of being used, as a measn of transportation on water. The watercraft need not have a trasnportation function so long as it is in navigation. A liftboat that you describe would fit within this definition and crewmembers onboard would qualify as "seamen" for Jones Act purposes.