Folks keep forgetting the first question. What is my mission? Then choose the best practical weapon for that mission. Many folks are one gun does all, so be it.
As to those that advocate a Hi-Point I have this little historical factoid:
At the battle of Waterloo, Major-General Sir William Ponsonby didn't want to risk his favorite charger. Instead, he chose a small hack he wouldn't mind losing. A French lancer rode him down and killed him.
This is what we call "false economy". --Unknown
The vast majority of my boating was done in the Ocean. So bobbing around, and swells were the norm, not the exception. Handguns suck in those environments. I would use a shotgun for anti-personnel purposes and other necessary missions.
Ideally a FULL STOCKED
, marine built or modified model. 20 or 12 ga. In my younger days, plastic shotgun shells were just coming on the scene, and the older and cheaper paper shells would swell when damp, and often be totally unusable if they got wet. So a dry box was mandatory, usually a surplus ammo can. No longer a big deal but today finding a quality dry box ain't a big deal either.
In the small amount of fresh water boating we did, Water Moccasins were the biggest and constant. Mostly my double barreled shotgun got it done.
None of the boats were worth a pistol/revolver. If I was hunting or hiking and using a canoe I would be carrying my model 19 S&W, or later the 6" 686, I like a 6" 357 in the boondocks to this day, even out west here. If hiking it would be primary, if hunting it would be secondary to a rifle or shotgun, depending on the hunt.
Don't loose to much sleep over this stuff. Carry what you got. If you boat a lot I would recommend a 20/12ga marine type shotgun. Remember to consider if you are going to other jurisdictions or not. Other places may not be as understanding as Florida, or where I live now Arizona.
What ever you get, learn how to use it on board it is NOT the same as shooting on land. It is much easier to get tossed/stumble/fall in a boat and shoot the bottom out. Seen it done several times. Just lucky it wasn't me.
PS Important fact, I am from Florida originally. Arizona was the compromise with my Los Angeles Wife after the Corps.