^ - A friend does that every other day in the SF Bay, year round, wearing just a Speedo. Am convinced his over 70 yr old body doesn't register cold.
When you are out of shape for the cold, your nervous system perceives cold at a big threat. Because it is.
One of humans (neglected) super powers is the ability to endure exposure to cold.
But cold adapting is a PITA. And takes conditioning.
Last summer I was doing ice baths in the low 40s.
I'm out of shape now where the cold doesn't bother me, but I don't freak out when I feel the terribleness of the extreme cold.
I can't wait until I fall into a frozen river.