You should also run an ad in the local paper describing the vessel. If you have the hull numbers, you can try calling the manufacturer of the vessel. If it is a small vessel which is drifting slowly by your dock, you should first decide if you want the vessel, and if so, pull her up on your beach and start using her. If someone shows up and says it is theirs, let them have her. If no one comes to claim her, you are the proud new owner.
Large vessels are a problem.
That brings back to me the memory of this old barge that did not pay her dockage. Somehow she got moved and was tied to a piling across the river from her old berth. The old girl got through four drawbridges all by herself. The Coast Guard was called by the bridge tenders and in turn called one of the local shipyards and asked for assistance in bringing her under control. They sent one of their yard tugs and towed her to their yard.