Or there is something called a "Game camera" at your local sporting goods store.
Usually meant to mount up (like in a tree, or in this case the mast) and snap pictures of whatever walks down a game
trail, or to the watering hole or....
These are motion sensor activated, and some are infrared and do not flash or scare the game
...or the douch-bag getting into your boat.
With caught "red handed" pictures, I am sure it would not take much effort to discover the bandits identity, and then...well, some things we don't discuss on the open interwebs.
I hate thieves even more than crooked politicians...of course I am repeating myself.
Good luck with whatever method you choose to end this charade.