I haven’t tried to swim in many years but back when I was young and fit, I sank . For a girl I was slim and muscular and always weighted more than I appeared to weigh. I always sank. I am fairly chubby now maybe I would float.
It’s amazing that just about anyone can lie on their back and float.
Yeah it doesn’t take too much body fat to make you float, so I don’t think many adults are going to have that problem, unless they are marathon runners or something.