I use a Nikon D40X on the river. I bought this model because it was smaller than most SLR camera. I didn't have an investment in old SLR lens, so I could go with the smaller DSLR format. It turns out the camera body size doesn't matter much - it is the lens that makes it big. But for rafting trips it works great.
I have a Nikkor 18-200mm lens which covers 90% of my needs. I also have a 70-300MM lens that I use for long rapids or when I can't get close enough. But the 18-200 is my favorite lens. It focuses fast and has a fast shutter speed.
Here are some samples from a Lodore trip a few years ago.
Picasa Web Albums - Dee - Lodore July 2007#
My only complaint is that the sky often comes out over exposed on scenery shots. It does really good in the sports setting, but landscape settings don't turn out as well as I would like. I am not yet very good at using the manual settings, but trying to learn.
I have been very pleased with Canon point-and-shoots and video cameras. I went with the Nikon because of the recommendation of friends, and the smaller body and the point-and-shoot modes that it supports. It seems that my Canon point-and-shoot gets better color on scenery shots, so I wonder if I would have been happier with a Canon DSLR. But I now have too much money invested in lenses to switch. I suspect they both have pros and cons. I also liked the feel of the Nikon better.