Half my platoon in boot camp was from Brooklyn or inner city NYC (mostly black people). When it came to swim qualification, it was apparent that these recruits could not swim very well. The cause I think is the lower body fat that others have mentioned and perhaps the lack of good municipal pools in which they could learn to swim. I learned to swim at my grandparents’ backyard pool. Grandpa thought it was funny to throw me and let me figure it out. LOL. What a cut-up he was!
That reminds me of a story my mother told me. It seems that her father had the same habit, i.e., throwing his kids into the water "to teach them to swim." Well, unbeknownst to him, one of his sons had learned to swim before and when his father threw this boy into a darkish lake, he swam under-water and came up in a location his father couldn't see. Scared the old man half to death.