In the Midwest there are polar bear clubs where swimmers enter a lake or river in the winter. Granted they don't swim, just plunge and hurriedly exit.
The guy in this story was swimming in 21 degree fahrenheit water. There are polar bear events when some years they're entering water thirty degrees colder than that! Nothing that I've ever been tempted to try.
If fresh water were -9 degrees F, I suspect it would be solid ice (it could be super-cooled if it were absolutely still, but lakes and rivers are not that, and splashing in super-cooled water would turn you into a popsicle). The air temperature isn't even that cold in any of these pictures.
Actually if you read the link you'd see one of the polar bear events the temperature was -8 degrees fahrenheit. I've watched similar events on TV where they actually cut a hole in the ice to begin the event.
The guy in this story was swimming in 21 degree fahrenheit water. There are polar bear events when some years they're entering water thirty degrees colder than that! Nothing that I've ever been tempted to try.
https://www.jsonline.com/picture-gallery/life/green-sheet/20...