I’m also struggling to see how one’s risk of drowning isn’t obviously related to fitness. (The nexus between weight and car crash mortality isn’t exactly buried science, either [1].)
Which further undermines the authors’ assumption that drownings are random accidents.
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.