I'm thinking about going on a couple of kayak excursions of the streams that cross the forested areas near where I live. A couple of them are well known kayaking destinations. Although I was quite an experienced canoer in my youth, I've never handled a kayak before and I know that even for experienced kayakers, an occasional capsize is not uncommon in some of the narrow passages where a run in with a submerged stump or overhanging tree limb can catch you up. One of my main objectives of course is to do some photography along the way.It is hard to beat a pelican case for water resistance, but they are particularly awkward to carry in a kayak. One big enough for your body and lens will not fit into the hatches of a sea kayak and for a river kayak... forget it!
My question is how to stow my gear when it's not in use so that in the event of a capsize, it stays perfectly dry. I would need some sort of water tight container that could hold a 5D III body, 70-200, and 24-70 lenses, but be small enough to not take up the whole boat.
You could try a "dry bag". You can find them at just about any outdoors store. Get a heavy duty one, they are a lot tougher than the lightweight hiking ones. put in a towel for padding and throw in a few desicant packs "just in case" and you can keep it in the cockpit with you.
I use a 5L one for my SX-50 and a 10L one for my DSLR