Bear in mind that PMR446 is voice only, and if your caught using it for owt else you could be facing a fine. How about one of those systems with a GPS and mobile that detects when the boat is moved, and sends the coordinates to you via SMS at regular intervals?
Of course, when it's on the drive, the microswitches etc linked up to a bloody loud siren'll do the trick. Alternatively buy one of them metal RIB type things and connect it up to the mains at night. Gives the kids a light show too.