My highschool had a required drown-proofing class in order to graduate. UNLESS you were a sinker. A few people in each class were sinkers (Michelle, in my PE class it WAS the two tall, very thin, African-American guys who didn't float). It has to do with muscle-mass and body fat.
The rest of us were taught how to rest face down in the water, legs dangling and periodically lift our heads for air. You use the least energy that way and last the longest in open water.
____
Melissa, 33 bronchiectasis (no CF)