Had a friend who was a C-21 (basically an exec transport Learjet) pilot in the USAF. African-American guy who pumped a lot of iron - heavily muscled- doubt he had 5% body fat. Most negatively buoyant guy I’ve ever met. Went through water survival refresher with him in the base pool. He never had been able to learn how to swim. His technique in the pool was to just let himself sink to the bottom and RUN underwater up to the shallow end holding his breath.

The instructor was duly impressed but opined that might not work in the Atlantic, or even the Med.