In a kayak, you have to hold on to your paddle which drips water on your lap, which is where your camera is, and when you put down your paddle to take a picture, the boat veirs off to the side away from your photo. with that said, I am planning to do just that with my kayak this season. I have been shooting my point and shoot with a waterproof cover for a couple of seasons and it just isn't very rewarding.
Instead, I am going to get a pelican case like this one:REI Pelican case
I will be attaching it to the bottom of the kayak with velcro. As some people said, if you are in a kayak, you WILL get wet. Everything will get wet. this is true as a rule, so cameras need special care. I will not take my good camera out unless I am in calm sheltered waters.
As for photography in a kayak, there are some great things and some terrible things. Since you are so low in the water, there are opportunities that you might never see in other places, but your point of view is limited and somewhat constrained. Unless you have a longer lens, your foreground will almost always be lots of water with lots of sky above. Polarizers help to improve images in this situation by both cutting glare and decreasing some of the dynamic range as a result.
Further up in the discussion, someone suggested that boats are always moving so that is a problem with slow shutter speeds. Add into that the fact that you need long lenses to take pictures of anything interesting and you compound the problem.
So what are cameras good for with kayaks? I think they are best for quietly sneaking up on wildlife, mostly water birds, to make their picture. I also think they are great for micro landscapes along the shore....little images where the landscape may be only a few feet across the waaters edge.
You may also want to check out the paddling.net bulletin boards for suggestions about boats to use for stability. Some kayaks are low and fast (also generally wet and unstable) while others are higher, dryer and more stable (generally slow and clunky). Paddling.net
Check here for some pics from kayakers on the water.Woody's Kayak Trip Reports
see you on the water.