Some people float fairly easily on the surface of the water. Some sink, sometimes well below the surface before reaching equilibrium. I’m like that myself and cannot remember being unable to swim going back to my earliest memories. I always envied people who could just lay back in a pool and float around with no effort. I can only do that in salt water, later in life after accumulating some body fat. The kids who sunk were slender and muscular, a more dense physique. The kids who floated easily had a layer of body fat, not necessarily obese.
I understand what you’re saying. I’d like to see some stats on relative body fat in children of different races. Btw, when I was a kid I couldn’t float either.