I've canoed since 1963, the first few years with just a Crosman 38T .22 caliber CO2 pellet gun, then a .22 rifle, briefly, and then always with a shotgun (during waterfowl season)and a Colt New Frontier .22. In all those years there was one instance where a snake tried to get in the canoe. Unable to tell if it was venomous or not, we dispatched it with a load of fours from a 12 ga. Another time I flipped the canoe (the only time ever in over fifty years of canoeing) in a rain-swollen river I had no business getting into in the first place, and the old Colt, in a full flap cavalry holster, survived fifteen minutes in raging flood rapids. Gotta love those cavalry holsters. A few years ago I gave up the Colt to my youngest son (the little snot has gotten real good at talking me out of guns), and replaced it with a Single Six in .32 H&R magnum, which has since been replaced by a couple of S7s in .327 FM. I hardly ever shoot anything anymore but the .327s. It's my new .22 and .357 all rolled into one convenient package. .357 punch, if I want it, in a .22 frame. And a single action to boot. What else could one want? I like it so much, I've done something I've never done with a handgun before, ordered and installed custom, hand made grips from a chunk of premium grade walnut I've had laying around for years. Pachmeyer and Hogue just weren't going to cut it. I am all about the .327 these days, and boost it every chance I get. Check it out in Single Seven (it's a seven shot six shooter, just like Handsome Stranger carried in "The Villian") or SP101 or LCR. BTW, Federal factory 100gr soft points do 1600 fps from my 4 5/8" S7 barrel, and 2241fps out of the 20" Marlin 1894CB reamed to .327 by Ranger Point Precision (thanks, boys it still shoots great). That's some respectable smack right there.
OK, deserted, you have quite a few good bits of info in there to talk about.
I didn't even think about a flap holster. It slows down the draw, but in a kayak that may tip it may not be a bad idea if I don't go with a gun with a lanyard. Of course, I've never had my kayak tip, so most likely, the main thing will be water getting into the kayak from the paddle (happens every time) and the gun getting wet (so stainless, polymer or aluminum is still important, but submerged isn't all that likely). Now, a kayak is more likely to tip than a canoe (I love canoeing too, but those I only rent), but it is far from certain in a kayak too.
I wasn't even thinking about .327mag, but that could be fun at the range, and make a good self defense gun when not out on the water. On the water, in a J-frame sized gun it will have 1 extra round v. the J-frame which is nice. I'd definitely like the Single Seven with two extra rounds vs. a J-frame .38/.357 or 1 extra round vs. a K-frame. Though, it won't provide the cheap practice or even higher capacity of a .22 when at the range.