The reason for post immersion shock is a reversal of the process that Freef is expressing so much doubt about. There is a centralisation of blood volume during immersion as Dave T and I have pointed out. At the same time, over a prolonged period of immersion there is dehydration due to an immersion induced diuresis (Freef, you might want to ask yourself why that happens too) and other factors. If the victim is then hoiked out of the water in an upright position there is a sudden loss of (a now depleted) blood volume into the dependent vessels (mainly the large veins in the legs) and this can be sufficient to cause shock and cardiac arrest.

So, for Freef's sake, let me restate that another way: you can be upright in the water with no problem. Then you get removed from the water in an upright position and you die of hypotensive shock. Think about that.

There is no magical time at which this becomes an issue, but the longer the immersion the greater the risk. Frank Golden's definitive work on this issue, published in the Journal of the Royal Naval Medical Service (in 1992 IIRC) unequivocally recommends removal of prolonged immersion victims in the horizontal position.

I understand that GUE is making a DVD of all the talks, with the power point presentations included. There are bits of mine that might have to censored, but the important bits will be available.