"I learned this from Mythbusters, in the episode about crashes (cars/helicopters) if you find yourself disoriented underwater: Calm people live, panicked people die.

I was in a boat crash a number of years ago, and I believe this saved my life. No flotation device, stupid, I know. I don’t remember the impact, I was suddenly several feet underwater and didn’t know which way was up. I freaked for a split second, then that advice popped into my head. I stayed still, and let buoyancy do its thing, saw some bubbles and followed them as they rose."