Interesting url about cold water:
A couple of years ago I capsized with my kayak in Vänern.
The water temp was 12-13º C. No drysuite, just shorts and a Tshirt. I was back up in the kayak in 10 minutes but it took a while that evening to get warm again.
It was a great lesson, I learned a lot that day ( after that I don´t have so much loose bags and things around my seat) I also purchased a drysuite.