A swimming instructor once told me that blacks have denser bones making them sinkers thus having more trouble learning to swim. Don’t know if it’s true but in my experience most blacks do not swim well, if at all.
I was taking swimming class at a local pool and the instructor tested us for buoyancy. The one black guy in the class went right under.
Speaking in generalities (there are always exceptions), Blacks do have a denser bone and muscle structure than those of European ancestry. Quite some time back in another career, I was a swimming instructor at the Recruit Training Pool at Parris Island. Every Marine recruit went in the water. Those who could not swim got a couple of days of instruction before testing. As one who was in the water with them, and sometimes going to the bottom to retrieve them, a far higher percentage of Blacks than Whites simply did not have positive buoyancy.
That does not mean they cannot learn to swim, only that it’s a tougher process. BTW, the second fastest swimmer among the instructors was Black, but even he could not float without arm and leg movement. (Modesty prevents me from identifying the fastest swimmer).
Race and genetics are contributing factors.
Less body fat and higher bone density do contribute the buoyancy of the human body. While all can be trained how to swim, beginning stages take more work for lean bodies with denser bones to tread water to remain afloat.
Additionally, water conducts heat far greater than air. The less fat layer on the skin allows the body to lose heat much quicker in the pool for the same muscular physiques.