Not sure about the legalities in your state.
But, I can say there have been numerous reports of guns lost in boating accidents. Why, on one river in Washington state, it sounds like the entire river bed is positively littered with all manner of firearms.
Curiously, reports of these accidents seem to skyrocket whenever new anti-gun legislation is being promoted. Another curiosity is that the boats never sink--just the guns. The boats always make it back to the landing.
So, your friend will want a storage method that keeps the gun securely on the boat.