For the past ten years, on Superbowl sunday people jump into the Atlantic Ocean off of Long Beach (Long Island) NY. I heard about it last year, and this year I decided to join it as well.
We lucked out this year since the air temp was ~46F & sunny, and the ocean was ~39F. I'll update this text if I ever hear the official estimate of how many people were there but I'd say many many thousands.
The event was great, except I was stupid and left my glasses on while I went in!!! And surprise surprise, they didn't come back out with me. :(
Official Site of the Long Beach Polar Bears: http://www.longbeachpolarbears.org/