One black friend of mine, in boy scouts, had so little body fat that he literally could not float/sunk like a rock even when exerting himself not to. It was amazing to watch.
I’ve also heard that in Navy SEALs training, guys with a modest amount of insulation can swim and endure the cold water, better.
This is common knowledge.
Regards,
Muscle and bone sink. Fat floats. See: Whales, Dolphins, Seals, etc. And perhaps a genetic ancestry of European winters’ survival led to a couple extra layers of subcutaneous blubber in we of melanin poor complexion.