Swimming in the Pacific Ocean, going through the surf and out to swim buoys and such, I would get hypothermic in about 15 minutes without a wetsuit in water that was 62 degrees. But then I had about 8% body fat and was in triathlete condition and absolutely HATED being cold and when cold, I quickly lost body temp.
One time, after a long ocean swim (about 3/4 mile) in 61 degree water, I was with a bunch of co-workers and trying to eat lunch, but was shivering so badly it was making everyone else cold just looking at me (I believe the SEALs call it "jack-hammering"). I EVENTUALLY warmed up, but man was my jaw sore from shivering so long and hard!!
With my body fat level raised CONSIDERABLY after hitting 40 many years ago, I no longer HATE being cold, but rather HATE being hot!!
How times change!!! LOL!!
