Wetsuits provide a basic layer of protection and are recommended even for warm water. Rock edges are rarely forgiving and unless you're in a very deep river, chances are you'll scrape against one now and then. Or if you're like me you will come into contact with one at full speed and it will leave a painful bruise. With a wetsuit at least the skin is less likely to get cut (although it still can), but with pads and a wetsuit, your chances of bruising are lessened exponentially.
For warm water and air combinations, a "farmer john" style suit with a rash guard might be enough. Your upper body is in the water less than your legs, so it makes sense to free up your arms. Wetsuits have come a long way in terms of flexibility and warmth. Here is a good review of some of the warmer wetsuits available.
My first 6mm suit for the Pacific Northwest surf was a zipperless Rip Curl. Being Houdini would help to get in and out of this thing, and after a faithful five years it is semi-retired and has some leaky spots. It's a good backup suit, when my newer Body Glove Storm Trooper is damp. It's a 5/4 (5mm in the body, 4 in the arms and legs). It has an internal zipper, so while I don't quite have to be Houdini, sometimes I have to get help finding the pull string.
For really cold water, I wear two suits; my 5/4 Body Glove with a 3mm farmer over it. While it's a lot more work for my hamstrings, it makes a huge difference for staying warm, and for keeping the pads in place, which I'll get to in a minute. When the foothills have snow, melting into the river, my 8mm body felt fine, but my feet froze. Check out the wetsuit selection at Face Level.
If you are in a really cold climate or water, a dry suit might be just the thing. Makes sure you get one that won't tear easily. Here is a stylish dry suit that looks like snowboard wear.Need help deciding how much wetsuit you need?