Meh, for short term submersion (the sort I've experience while rafting at least) a 7mm farmer john top and bottom would more than suffice. Of course, if you're going to be spending more than a minute or two in the water something heavier would be nice, or a dry suit. People here sail dinghies in water at or below forty degrees in just farmer john bottoms, they fall in, right the boat, and are in the water just less than a minute. Of course, there are crash boats in that case, so the top isn't as important.