For whatever it's worth...

I spent years as a member of a Dive / Rescue team in an inland recreational
waterways enviroment.

We never recovered a drowning victim who'd been wearing flotation.

We used to say that we'd never recovered a body that wouldn't have been
alive if s/he'd been wearing a PFD, but then a carload of kids drive into a
partially frozen river. I don't think even PFDs would have helped them.

We were all strong swimmers. We could tread water for long periods with our
legs tied together or with our arms out of the water. Yet NO member of our
Team ever went near the water without flotation of some kind. NO
exceptions. Often we'd judge the probable area of submergence and the drift
of the current by looking where we'd found the mandatory life jackets.
Floating on the surface. After the accident, when it was too late except to
tell us where to put the first diver into his search pattern.