This question was prompted by PaulD's question about hypothermia prevention after a falling into icy water. A subsequent answer by Roddy stated that the immediate danger of cold water immersion is not hypothermia, as it takes quite long to lower the central body temperature. This statement caused some controversy on how fast this really happens and I feel like it is not what was asked by the OP. So here it is:
How long does it take after immersion into cold water to develop hypothermia?
I am interested in different water temperatures and without or with some kind of heat loss protection (relevant for diving/sailing?), just state for what conditions your information hold.
It is generally considered that the first stage of hypothermia begins below 35degC core temperature. Please reference studies, first hand experience or give any other sort of explanation, why your source is viable. I found many articles via google that claimed different things, mostly not well based (at least not declared).