I had a mild hypothermic episode this morning, I can't recommend it. I've had a bit of a cold all week and hadn't swam much but I really needed to get into the cold water today. Did 600 metres in 5c and didn't feel too bad, got out as usual, sat down for a few minutes to prepare for the after drop. But walking towards the sauna, my heart did not feel right and my body felt odd and all of a sudden I felt like I was drunk, so I sat down again outside the sauna and it got worse and I found I couldn't think straight. So I went straight to the lifeguards office, told them the situation and they stuck me in a dryrobe and sat me next to the radiator. A cup of tea and 10 minutes of uncontrollable shivering later and I felt fine, went into the sauna to fully warmup.
They were quite impressed with how sensible I was, they tell me that often they have to literally put people in wheelchairs after they collapse!
Moral of the story, listen to your body especially when swimming in water this cold and don't swim if you have a cold as I think it puts too much strain on your heart.
It's amazing how your heart seems to set the equilibrium for your body. I'm fine now, better than fine :D