Here's an idea.
Take a trail camera with an infrared flash, hitch it somewhere inconspicuous with a field of vision on the approach to the boat. Could be an outward piling, dress it up like a hunk of old tire, somewhere hard to see and hard to get to, because if they see it after they do their crime, they have to try to steal it or break it.
This way at least you get a shot at knowing the next time (hopefully there won't be) that someone F's with your territory.
Amazon.com: Wildview Infrared Digital Video Recorder with 24 IR Emitters, 32 MB Memory: Sports & Outdoors