There's a good article in the July edition of "Touchdown" Magazine on use of immersion suits, see link below....
http://www.navy.gov.au/w/images/Touchdown_July_2009.pdf
In the RAF we were supposed to wear them when the sea temp was 10 degrees C or less and at other times when rescue might take longer than the predicted survival time due to local SAR deficiencies....
In the 70's there was a two-piece white (when new!) acrilan pile thermal "undergarment" later replaced by a one-piece green fleece "bunny suit". This was worn in addition to cotton long-johns and long-sleeved cotton T-shirt.
It made you quite "bulky" and warm......
MB