Well, my immediate concern with a DSLR is water...so I'll assume you're kayaking in lakes or other suitable calm water. That being said...I'd use a quick-release tripod mount and keep the camera in a hatch bag (one of the little canvas bags that fits inside the hatch lid) and keep a monopod under my butt. That way one could access the camera quickly, snap it to the monopod and have a flexible, lightweight and fast system for using the cam, and a relatively safe/dry storage for it. As long as you're not using a long zoom and the body/lens will fit in the hatch anyway.
As for mounts, I use the railblaza system and mount a number of different things in different places on my boat, including a gorillapod arm with a tripod bolt epoxied in it, to mount either my gopro or my WG-4 point-n-shoot. Probably not ideal for a DSLR's weight, but you could certainly use the system (or the RAM system) to rig up a shooting platform or gear mount. The railblaza system is a bit easier to change stuff out that the RAM system, but otherwise it's personal preference.
If you are talking about the ocean...get a water housing!
2013 Tandem Island "Victory Lane"