I taught kids and some adults to swim from high school through college. Black kids can swim as well as anybody else, but there are some barriers to overcome.
First, getting them to submerge their faces and get their hair wet, definite bias against that. Once you get past that bias and teach them how to keep water out of their noses by gently exhaling and blowing bubbles, it gets better.
Secondly, there is a difference in buoyancy between slender, muscular kids and kids who have a layer of body fat. Being able to float means it’s much less scary when getting into deep water for the first time. Sinking below the surface creates panic. However, everyone floats, reaches a state of equilibrium. Some who are slender and more muscular will sink below the surface of the water before this is reached, sometimes a foot or two beneath. The survival float, or with kids, the “jellyfish float,” enables them to relax and realize that they’re not sinking to the bottom forever, that there is a way to move and to act in the water that works without having to try too hard, no reason to panic.
After that, some can be taught to be merely proficient swimmers, some good, a few great. White kids take to it more easily as a group but there are outliers who panic, sink, don’t want to get their faces and hair wet, etc. Black kids take to it less easily but there are outliers as well.
I’m skeptical of the body fat explanation. We were a bunch of skinny white kids when I was growing up and learning to swim. I’d be surprised if black kids these days are statistically any leaner than we were. I do remember some of the black kids struggling but it seemed related to a fear of water and a tendency to panic. I think they were acculturated into that fear before they had any experience with trying to swim.
You're kidding yourself. Buoyancy is a simple scalar property. If anything, you lose buoyancy as you sink due to compression. There is no tendency to become more buoyant with depth.
I myself am near enough zero buoyancy that I can float, then expel air and sink. This is how I do my "lobster imitation".