Here a couple of ideas.
1) mount a video camera so you can record the boat number of the theft, these are expensive though and require electricity
2) at least get you a motion activated game camera from bass pro outlet store for $100 normally used for deer, but I bet it would work here as well
3) add a little electricity around that filler cap
I bet if a few of em get shot the number of incidents goes down
As for not being able to leave a boat on the lake anymore that just plain sucks and I don't think we should have to live like that. Constantly in fear of and bowing down to criminals. Load up boys.