I would disagree with the biometric safe idea. Non-marinized electronics
dependent. Not a good idea for a boat in my opinion.
If I were to put a safe on my boat I would go for a quick combination, mechanical only safe. Something like the ShotLock CCW Solo
(as others have said, no gun discussion just talking about the safe). Its small and can be through-bolted with backing plates
to make it secured in place well.
As others have pointed out, I wouldn't count on it not being found. I would have some backups for it like hiding money in other locations just to be sure it's safe. I also like the idea of having something sacrificial that would look like your valuable possessions. Maybe a cheap
lock box with some costume jewelry, a little bit of cash and expired credit cards.