To piggyback on B-the USMC taught drownproofing as a part of SEAR (survival) school (back in the early 70's). We also had the 6 hour "final exam". The basis of using the "deadman's" float is that one can float at a natural buoyancy level and use the absolute minimum energy to raise and breathe. One funny note-we did ours in a 30 foot deep pool, had to jump off a 20 foot tower in full pilot gear, get gear off after hitting the water and float for six hours. We had one guy who just had no, as in like zero, natural buoyancy. The only thing that stopped him from sinking was the bottom of the pool. Because no one had ever seen that, they actually had some Drs down from Bethesda Naval Hospital to examine him and they figured out he had the densest skeletal structure they had ever seen. On the flip side, we one guy like B, he floated with everything from his shoulders up out of the water. He probably could not have drowned if he wanted to. On a not so funny, but ultimately good note, we did have a pilot go down about 100 miles off NAS Jax. He spent 32 hours in the water before being rescued. Other than pretty substantial dehydration, he was in amazing shape. And I can confirm what B said, floating for 6 hours is really, really boring!