Many years ago I taught drownproofing to poor kids -14-17 yrs old.
It was hard with blacks. Many black males were "sinkers", that is with a full breath, lying motionless in the water, they sank. Very few whites are. Even some black females sank, whereas I don't think I ever seen a white female sinker.
I don't know the reason for the difference but it is pretty obvious to anyne who has taught swimming.
ROTFLMAO
People of African origin have bones that are more dense and a lower percentage of body fat. People of Irish descent, ditto.