My university required a swim test to graduate, and also required 10 minutes of treading water in order to access sailboats. Many of my classmates talked about this being difficult, but both were trivial for me.
Fat is buoyant.
Fat is buoyant.
That's probably confounded. Anything over a BMI of 23 better be an increasing proportion of muscle, and even then there's a point that the stress on the heart isn't worth it.
Almost every physical and mental heath condition does bad things to nutrition and internal energy stores, even if only at a diet level.
It's hard to see since so many people are overweight or obese to start with, but the overall correlation goes that way enough to cause confounding.