I think the only way around this will be to very gradually add the boat onto his list of safe zones. I have thought about ensuring he has his own bunk that doesn't get shared with anyone else as well as at least a drawer that is his to store whatever he wants and doesn't get touched by anyone else. As our first sailing vessel will most likely be a trailer sailer, I guess we could organize spending a few nights camping in it parked in the driveway.
Brilliant! A Hartley or similar may be exactly the thing.