I really think I'd put some sort of sign like "Fragile- please don't sit or jump". People have no idea.
A sign like that will just encourage people to sit on your boat.
Next time you see someone sitting on your boat, go plop your butt down on the hood of their car and ask them how they like it.
In all actuality though, I think this type of behavior has to be anticipated (though not necessarily accepted) when you leave your boat on the beach. Unfortunately most people consider boats on the beach to be a piece of the beach and part of the public domain.
sm
