I hadn't really given such much thought as a kid, until in Marine Corps Boot Camp we all had to jump in the pool in full gear.
This was the 60's, and the atmosphere wasn't so politically correct, so the pool instructor got away with what he said as we prepared to go in:
"Watch the n*ggers! They're all going to the bottom! They're 'Sinkers'"............damned if he wasn't 100 percent correct.
Actually, it's not.
I knew many 'white' people who claimed they could not swim. That they would sink like a rock.
Since I swim like a fish, this seemed weird to me. I know for a fact how hard it is to sink to the bottom and stay there. One must wear weights to do so (or just let all the air out of your lungs).
I was trying to teach a friend how to swim and he said his 'skeleton' was heavier. So... I grabbed a 1-gallon can filled with concrete that we used as an anchor and jumped in. I swam around with it , showed him it made no difference.
Then I watched him try to swim, and I figured out what he was doing wrong.
Those who claim "they can't swim" are trying to keep the water at CHEST LEVEL. This is impossible. They fight it until their arms tire and then they start slipping under.
I taught him that he must accept that the water level will be at his CHIN. A simple tilt of the head keeps the mouth out of the water. One cannot 'float' any higher than CHIN LEVEL without extreme exertion.
I agree that most 'blacks' cannot swim, but it is because most of them never try, or were never taught properly, or never grew up in an environment where they learned naturally. There were not that many swimming pools in the inner cities of our nations. Sometimes I think it is a mental limitation (artificially imposed) that comes from watching TV and seeing only TARZAN in the water, and never the natives. (Besides the fact that the waters in African nations were full of gators).
At any rate, there is no physical reason that those who claim they can't swim (black or white or green or red or blue) cannot actually learn to swim.
It has absolutely nothing to do with body size, body density, physical build, or race.