Back in the sixties I saw a documentary about submarine escape testing methods using the Naval 100ft deep water tank. As an example they asked two men one black the other white to jump into the tank and not to move a muscle ...first in was the white chap ... He bobbed up and down bellow the surface. .... next the black chap. ... He sank like a stone. They concluded or thought that this was because of heavier bone mass. Maybe this is why we don't see many Olympic Swimmers. However I believe that this father ( God love him ) was a non swimmer.