It's the air inside the insulation of your clothes (e.g. fleece) that provides the real barrier against heat conduction. If you get the clothes wet, you will lose heat fast. Watch Bear Grylls on some of his videos in places like Siberia. As soon as he gets out of the water - he rolls around in the snow to dry his skin. Then he puts on fresh dry clothes.

I have never immersed myself in ice water. But I have been pretty soaked in the mountains and seriously chilled out. If you don't have some fresh dry clothes to put on pretty fast - then you better have another source of heat ... like a good roaring campfire. Plastic bags and mylar are not going to do you any good if your body is already seriously chilled. You'll get hypothermia really fast.

I suggest that you go and experiment. It's still cold in most parts of the country. Pick a snowy area beside the road, or do it late at night. Either way - leave your car running and the heater on inside. It would be smart to take a friend with you :-)

cheers,
Pete #2