Cold water swimming is something I've recently come to enjoy dealing with, as a frontier that could be explored late in my life (early 70's), when physical/medical limitations preclude other avenues of outer-limits challenges.
I'm as surprised as the folks who know me that it IS possible to swim in very cold water. During the past 4 years I have swum in the Atlantic Ocean several times in most winter months, here at the New Jersey coast, managing to swim 1/2 mile in 54º water on one occasion (12/4/15), and 1/4 mile in 44º water on another (11/25/13). Although I have missed a couple of months here and there, due to injuries, illness or rough surf conditions, I have done many shorter cold swims, including 150 yards in 35º water on both February 23rd and 24th in 2014.
No wet suit. Just a thinly insulated cap. (I'm basically bald, so I have no natural insulation factor that younger swimmers get with having hair between their head and the cap.)
How do I do it? Mostly, I follow Terry's philosophy about concentrating on swimming thoughts during every stroke, rather than letting my mind be distracted by thinking about the temperatures...
As fascinating as the cold water swimming is, I don't sprint for the towels and warm clothing right after coming out of the water - rather, I actually enjoy walking 1/4 to 1/2 mile along the beach in the winter breezes to dry off first. And, I like the air temperature in the upper twenties much better for this than in the mid-30's. I am conjecturing that the air below 32º loses most of its moisture and is more comfortable than the just-over-freezing moisture-laden air.
One reason for pursuing all this is that the ocean is often very calm, sometimes even as flat as a lake, during the winter months here when the prevailing winds come from the West or North-west and blow the waves out to sea. Which means... one can swim smoothly and safely in chest deep water along the shore if desired, rather than having to go way out beyond the breakers.
Winter swimming at Loch Arbour, NJ - cold, but CALM AND SMOOTH!
However, sometimes the water has defeated me - twice this March, just a few days after having done some successful short swims, I tried to go into approximately 50º water: my legs screamed with pain - I could NOT force myself into the water. This happened 2 days in a row. I have yet to figure out what was going on with my physical and/or mental status on those two particular days when my body "just said NO".