I know this is an archery thread and someone suggested we stay on topic, but I just had a cool little brainstorm: hand weights. I.e. small dumbells. Like 2 or 4 lbs, with non-slip coating.
Unlike a bat or even a knife, someone is going to have a hard time taking this from you, because there's only one place to grip it and you got that. And if you are in reasonable shape, hitting someone anywhere on their body with these is going to be very debilitating. It's not going to get noticed in your hand like a knife or bat, or gun, or bow. Obviously this is a close quarters combat deal and not protection against someone with a gun (although in the movies you would just use the dumbbell to deflect all the bullets, right?).
You could have several of these around the boat and they aren't going to draw attention from any gov't official. Would be a lot better than nothing.