I jumped in the Winchuck River once when it was mostly recently-melted snow. I was a teenaged kid then, it felt like someone hammered a 16d nail into the crown of my scalp. And then- it started *really* hurting. I stayed in, mostly because there was an attractive female classmate there and I didn’t want to appear weak.
Can’t do it now- heart issues. The body will do whatever it thinks it has to to preserve core temperature, which usually means sharply contracting major blood vessels. No thanks.
I was on a whitewater rafting trip when I was in my 30s and in good shape. At one point, we had stopped for lunch and the guide told us that we could jump into a race on a section of the river. Of course, I jumped in. The water was so cold! My breathing became labored and thought I was going to have a heart attack. I did not hear anyone else in the group, most of whom were younger and male, describe having an experience like mine.