Perhaps he should skip the handgun and carry what most gun "experts" agree
is the best thing for defense: a 12 gauge.

A long barrelled shotgun might work very well at close quarters on deck, but in
the confines of most boat cabins that long barrell would be an impediment. What
good is a weapon 4 or 5-feet long in a cabin that's 10 feet wide? Two steps and
your target is closer than the muzzle. Oops.

Saw it off down to the stock, and when you go for the kill make sure you aim
*above* the waterline.

Or, boat without a firearm. Most folks do,
and if you're just doing recreational cruising in US waters it's a real stretch
to come up with a compelling reason to be armed. Still, it is your right to do
so and if you're so inclined- enjoy.