I don't - I'd rather eat my own knees - but Mrs MCH has been doing it for a few years now. Not in the sea though as we live too far away. It's local lakes, pools etc and some local rivers with 'plunge pools'. She works on the basis of a minute per deg Celsius at this time of year. When it gets warmer again and she can put more time and distance in she's training for Coniston and/or Derwent swims.
Wow - those will be seriously demanding swims. Time to chew down on your knees and keep her company ;)
Edit: You may or may not want to watch the video that mondie linked to in post no18 - you just don't know where a Lake District swim is going to lead to..... (BTW, the video is terrific)