If you were wanting a long gun, then that is an option. Go with a stainless version of whatever you choose. I'd vote for a Stainless Marlin over any shotgun. This is due to several reasons, but the main two would be that the ammo is more compact and waterproof than shotgun ammo.
But, just because you take a long gun doesn't mean that there is no need for a handgun. As I see it, the long gun will, by necessity, be packed away, probably wrapped up in other stuff, eliminating quick access. Would you unpack it each time you hit the shore for whatever reason? You just KNOW that's when you'll suddenly need it !
The handgun, on the other hand, can be worn while paddling and it can be accessed very quickly. The gun that would work best in a kayaking scenario is easily the Glock. It's very durable and it doesn't care about being submerged.
I suggest a Model 22 (15+1 .40 caliber) a Model 21-SF (larger frame 13+1 .45 acp) or a Model 20 (15+1 10mm)
If you're new to handguns, or if you're going to buy your ammo off the shelf, forget the 10mm. Most current 10mm loads aren't that much more than the .40 loads. (You can buy some hot ammo online, but it's pricey and hard to replace on a trip. Hell, finding ANY 10mm ammo on a trip might be difficult!)
If you can rent a G-21-SF and G-22, shoot them and pick the one you like best. But find they kick too much, then go to the Glock 17, a 17+1 9mm.
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