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I've used an Advanced Elements AirFrame (inflatable). It's really slow and a pain in the lumbar, but the flat bottom and wide beam make it super stable. It packs into a suitcase-size case, which let me take it to Costa Rica. The main thing is that kayaks are much better than canoes in a current, since a canoe would be rear-driven, and the bow can get really squirrely. If you ever get a chance to test a bunch of boats, take each one out and tip it over; the harder to tip, the better for shooting. There are sometimes high-volume versions of boats with wider beams for larger people.
I may actually have the same Ortliebs, but never tried on the water... but may. I used a deck bag, dry bag, Igloo soft cooler.... I think a good, over-sized dry bag works the best, 'cause the camera needs to be accessible at the right moment. I also use a skirt when I paddle out, especially paddling through surf (Southern California).
There's lots of good advice here. 40Driggs is totally right about the drifting; I keep a paddle blade in the water to slow myself or give a slight push, but otherwise just drift in a low profile. I definitely use a leash on the paddle so I don't lose it while I'm shooting. I usually keep the left paddle blade in the water, the shaft under my left arm and held in my right elbow while I shoot. This acts as a sort of stabilizer for the camera, and I can make minor course-corrections at the same time.
Taken in the canals of Tortuguero, Costa Rica with D200, Sigma 100-300mm f/4: