A friend of mine who owns a gun shop does a lot of boating in the Caribbean. He says most people down there sporting nice boats keep a stainless Mossberg JIC under the seats. Pirates are a real concern to them. Most of their boats cost more than my house.
For me, when I do rarely carry on a boat, it's usually something like the 642 with snake shot. I'm generally in a canoe. I'm not concerned about most wildlife as I'm grateful for them graciously letting me quietly paddle by thier homes.
However, cotton mouths can be aggressive when you get near thier turf, and they can swim FAST. I'm happy to let them be and rage against the oar as I try to get out of their in a hurry, but I feel better having a couple of shots if I need them.
If I were shopping for a canoe gun that involved an overnight camping stop, I would probably go with a Glock 42 or 43. Small, light, thin, and about as waterproof as a gun can be as it strips down easy.