^^^ When I kayak, I use the type of drybag that is shown in the picture at the link djrichie provided. You put the camera in it, roll the top down twice and do up the clip and it's sealed. You put the bag under the bungees at the front of the kayak (or I suppose you could put it in the kayak, under the skirt, but that makes it that more difficult to get to it quickly). When you want to take a shot, stop the kayak, get the camera out and shoot away. I've also kept the camera around my neck and just tucked it under my lifevest as I've paddled, but since the lifevest is supposed to fit snugly that's not a great option. And I'm obviously speaking of calm waters and a dry day...you wouldn't keep the camera out if it's wavy or raining or you're in white water.
If you want more info, you can PM me.
Some people use the pelican hardcases, but they're more fiddly if you want to take pictures while kayaking. They work very well for just transporting the camera when on/around water.