My wetsuit is good but relatively light, and I was testing it under controlled circumstances. I'm a river kayaker, a sport which involves working hard in warm air before suddenly getting plunged into a cold river. It's the sudden transition that can cause cold shock:
http://en.wikipedia.org/wiki/Cold_shock_response
http://www.seakayak.ws/kayak/kayak.nsf/0/9ABAF7D7992B8205852...
The second link mentions cases of sea kayakers who were dead within seconds of overturning. Cold shock happens much faster than hypothermia.
Fortunately, both of my misadventures happened under controlled circumstances with a backup plan. But in both cases, I was surprised and scared at how quickly I lost my ability to swim. The interactive video we're discussing actually captures the feeling pretty well.