“but a friend who was a scuba instructor told me that from his experience, blacks do not float well.”
In my younger days I was the instructor for our community’s water safety courses. IO trained the cit’s lifeguards.
Part of the course was called survival floating. As kids we called it the “Dead Man’s Float.”
Basically you just hold your breath and go dead in the water with your arms and legs hanging down, face under water. You breath with as little effort as possible and try to stay as calm as possible. You can hold this position for hours if needed.
Everyone finds their natural buoyancy. Most people end up with a part of their upper back above water with neck and back of head. Some people it’s just the top of your head.
Black people often floated with the top of their head a foot under water. That was their natural buoyancy. It made survival floating much more difficult for them because it was an effort to get to the surface for each breath while other people could just breath with a slight swish of their arms and lifting of their chin.
This difference certainly doesn’t keep anyone from swimming, but it does make it harder.
Sounds like a good way to drown. ;-)