When I was young, which I realize is about the same time John Paul Jones joined the Navy, I frequently dover under the ice at Beaver Dam Quarry, which is now Beaver Dam Swim Club. The only protection we had back then was a 3/16-inch thick wet suit. We were good for about 30 minutes, then the cold set in. I later went to a 1/4-inch wet suit, which added about 15 minutes to the time. Of course, I was young and dumb at that stage of life, but the suit did a great job. A few years later I tried the same dive with a dry suit over the wet suit, added another 15 minutes, taking the total dive time to an hour. Ironically, the water was warmer once we got below 25 feet, but only a few degrees. The thermocline essentially reverses during the winter months.
The coldest dive I took was while stationed aboard ship in the North Atlantic near Oslow, Norway. After just 15 minutes in the water wearing a Mark5 rig and full suit I thought I would freeze to death. Checking the zincs on the ship is a routine job that I did monthly, and beieve me, it could be a nasty, dangerous job.