(Please excuse my use of Imperial system measurements)

I believe it was the British Air Force (or Navy?), in a post-WWII study of downed pilots, that formulated the "50-50-50" rule: in 50 degree water, 50 percent of your victims will be dead in 50 minutes.

But open water swimmers can swim for hours in water in the high 50s to low 60s. There are a lot of adaptations associated with regular cold-water training: mitigating the shock response, changes in heart rate and blood circulation, brown fat deposition and raised core temperature to name a few. The heat from muscle exertion also helps keep the body warm, especially the core. And just like most other sports, open water swimming selects for certain genetic characteristics. (Think basketball and height.)

Also, FWIW, it's one of the few sports where women compete equally with men.

Of course I'm talking here about water in the high 50s - I'm not sure how any of this applies to temperatures in the low 30s. But Lynne Cox swam for over two hours in 42° water when she crossed the Bering Strait. Most of us would have been dead in a matter of minutes.