Gazza, a couple of options, you could always dig a big hole under the boat, concrete it in and set in two heavy wall but removable steel posts that go up and through the trailer and locks the trailer frame in place, add a protected locking device to the posts so it can't be removed unless you uses a grinder, which would make lots of noise. Most thieves would not bother trying to cut it, too much effort and trouble. Hard part would be work out how to make a locking device that only allows a key to unlock it but not easily cut with a 18V grinder, and also how to make the steel posts easily removable once it is unlocked.
It would also mean that you would have to park it in the same spot each time. If you ever move you could just dig out the concrete and fill in the hole.
If too much work, a simpler alternative would be a sensor switch with a big loud siren and a timer, not talking about a motion detector but one that detects if the boat trailer is actually being tilted up or down to hook it up to a vehicle. Just some thoughts.