This was a charity plunge, see KeepWinterCold.org for details.
I'm glad I did it. I don't think I want to do that a second time ever. It was so cold on the beach the water actually felt balmy at first. When it hit my belly button, however, reason kicked back it. My brain turned to autopilot (because it was obvious to my primal self that all centers of logical or rational thought had malfunctioned) and I had to turn around. Part of me was a little mad at myself for not being able to totally submerge (despite warnings from the event organizers that first-timers should NOT submerge completely)... then I concluded I didn't really have a choice in the matter. Once my survival instinct kicked in, I don't think I would have been capable of doing anything other than scrambling for shore and clothing.
For the record, when we left the house it was 17 degrees Fahrenheit with light winds of 7 mph and overcast skies. By the time we reached the tents (which were not heated as promised) my legs had turned this interesting boiled lobster color and the tips of my toes were white. Now, warm in my house, my legs are white and the tips of my toes are red (and still a little sore).
...and all I got was a lousy t-shirt... ;)