When the air temps and the water temps added together are less than 100 degrees, there is severe risk of hypothermia associated with any kind of dunking.
This is the annual reminder to dress for the water temps rather than air temps (in general, of course), use dry clothing (I use a dry top and waist high waders, a drysuit would be better), have your bailout bag with you and be sure it has fire starting materials in it that you know how to use when your hands are shaking, keep high-energy food in it, and a spare set of clothing, plus maybe some form of instant shelter (I keep one of those astronaut blankets). If your water has rated rapids, add a full number to them for when you consider what you can handle (i.e., if you're usually a class 3 paddler, consider what would normally be a class 2 to be a class 3), don't go alone, etc.
There's a million other things to consider, but the point is ... be prepared to get dry and warm fast if you get wet, and assume that you will get wet.
- Big D