It's on the 'low end' of survivable [(20–28 °C (68–82 °F) Stage 3, 	Unconscious, not shivering, 	Severe Hypothermia)](http://en.wikipedia.org/wiki/Hypothermia), and in my experience, 72^o is when water starts to feel uncomfortably cold, but is still warm enough that with constant movement you can spend a very long time in it.