I recall many years ago watching an Atlantic salmon fishing program hosted by the venerable Hugh Falkus. He demonstrated the effect of waders in cold water by jumping into a deep pool on a highland river in early spring. By the way, he wasn't wearing a wading belt.
The lessons learned were:
1) If you get dunked DON'T PANIC!! Easier said than done but critical to getting out alive. The additional weight of the waders when full of water is actually slightly less in water than on dry land.
2) Normal swimming techniques when wearing bulky fishing gear may simply not work. If you go over in a river, try to float downstream on you back - feet first - to avoid getting a crack on the head by rocks.
3) When you reach shallow water, don't try to stand up - crawl out on your hand and knees. Full waders weigh a ton ON DRY LAND.
4) Hypothermia is a killer.
I'm fortunate in never having had to take an unexpected swim but practicing these types of survival techniques sound like a very good idea. Smart kayakers practice rescue and survival techniques. Roop mentioned trying to swim in waders a pool. Practicing getting back into a kayak whilst wearing all your gear also seems like a good idea.
Maybe a survival techniques thread would get some interest - anyone know of any good links?
If I could afford the expense I would definitely go for one of the Dry Suits which serious yakkers seem to go for.