If you can afford $20./month (+ the $300 for initial cost), this is quite a product. It works for your boat and your children (which would be worth it even without the boat)
http://www.spyassociates.com/g...2fcd3
There may be other systems that do this as well, this just happens to be one I ran across.
Also, if you have a double axle trailer, I used to wrap a heavy duty chain (one that could not be cut with bolt cutters) in between the two wheels and lock with heavy duty lock. Just a little extra protection. It'll still go down the road, but they'll be squeeling and skidding all the way, which may wake you up in time to follow them. Also, I know if I saw someone driving down the road like that, I'd be pretty suspicious and call the cops.
Hope these might help someone.