I had some snowbirds visiting in my boat one December. The water was clear, so on impulse I dived in the Gulf to grab a whelk off the bottom to show them. I swam around for about 15 shockingly cold minutes and then climbed back in the boat.
Uncontrollable vigorous shivering kicked in as soon as I did. It was the weirdest feeling. Lasted about 5 minutes even with towels wrapped around me.