I've never been hit on the boat(s) but I have known a _A_Lot_ of people who have.
I did have someone take some gas from my nissan
truck once though.
it didn't have a locking cap.
They crawled under the truck and pierced the rubber hose because that was easier than siphoning.
Cost me $400 to repair the damage for the $20 worth of gas they took. Same truck, they smashed the window of an UNLOCKED door to steal a gps
. Again cost about $400.
Locks by themselves don't actually prevent theft. It's like the saying that when faced with a grizzley bear you don't have to outrun the bear. you just have to outrun your friends.
If your boat is harder to pilfer than the guy next to you, then guess what?