IMO the total key to learning how to swim...or for that matter just how to float for extended periods is learning how to relax in the water. I've pulled people out of the water that were so tensed up they became cramped. Relaxed muscle floats...tensed up muscles sink. Even if you were tossed into the water by loving teachers...the fact that other people were there to assist you helped you to relax. Relaxation and slow breathing are the key. After a competitive period of long distance swimming I had a doctor doing a pulmonary analysis of my lungs for wearing respiratory gear in toxic/radioactive atmospheres tell me that my breathing pattern was similar to a whale. Huge lung capacity with slow breathing cycle. I think that was a learned property.....given of course your lungs were in good shape......like being a non smoker.