Sarah-Jane Prew the author of Survival for Aircrew says in the book, i quote "try this in a swimming pool. Get in one of these bags, fasten it over your head and float as still as possible in the survival position. Keep still for some time and you will probably begin to feel a bit cold. Then remove the bag and as the body heat rushes out and the cold water rushes in, you will realise just how warm it kept you."

From my experience of trying this with a normal orange survival bag in Chichester harbour, I had a similar experience but I cant say how much more effective the Land/shark is.