In my early teens with a group of friends we broke the ice on a local river and swam in the ice cold water which was something of a shock to the body for minute or two.

Never have and never will (voluntarily).

i hear the ocean is actually warmer than it is in, say, May, so there's that.
around here they do it for charity, get sponsors, then go out fo lunch & drinks with their supporters/handlers.

Dude, I don't even like walking to my car in the rain or snow, even bundled up. The fuck I'd ever jump my ass into freezing water.

Nope, never going to happen with me. I can't even stand plunging into 70 degree water, even with a wet suit! I always wonder if it's true that a polar bear type plunge could cause a heart attack? Even if that's not true, I'm not doing it!