A good plan is to start painting the boat. The owner will normally wait until you are nearly finished then pop up out a hole in the ground!
On a serious note i was once involved with moving a 36ft motor boat, this had been abandoned for 20 years & was disposed of by the local council legally.
A series of recorded delivery letters to last known address plus legal notices afixed to the boat advertising disposal, advertising disposal, the works.
When we moved her for the new owner we had literally just arrived back at the yard when the police swooped on us saying you have stolen Mr Smiths boat!
Once it was all explained they were happy & went away. It turned out "Mr Smith" who had abandoned it for 20 years lived in a house directly across the road from where the boat sat for all that time. Go figure!