[Anglika]

I had a go at this but have had problems getting it back.

Here's the first few paragraphs with my suggestions:

Physical activity in a cold environment

Vaclav Zeman
Institute of Sports Medicine, Medical Faculty Plzen, Charles University Prague,
Czech Republic

Cold stress is any environmental condition that causes a loss of body heat leading to homeostasis. Two major cold stressors are cold air and water.

Man is a typical tropical animal. This is confirmed also due to the critical temperature. Critical temperature is the lowest ambient temperature at which a nude man sustains his body temperature at rest without raising his metabolic rate. The greater the thermal gradient between the skin and a cold environment, the bigger are the heat losses.
Critical temperature in humans for air (Tac) is 22 - 27 °C, for water (Twc) 32 - 35 °C. Cold-adapted people have lower values.

Survival time for shipwrecked men in different water temperatures (Tw) based on reported experience from the 2nd World War (Thauer 1965) indicates that the corresponding time for 14°C is 8 hours, despite the fact that cross channel swimmers can be swimming in this Tw for as long as 13-20 hours. The explanation for this consists in the different physical and mental condition of swimmers and in their adaptation to cold.

Is it better to swim or not in order to maintain body heat?
The advantage of swimming consists in the heat production. However swimming also has disadvantages. It produces vasodilatation in the body surface area and in active muscles, and the cold water streams along the body. Hence the body cooling is greater.
In what Tw is it better to swim? In most people Tw 18 °C forms the borderline. Over that, it is better to swim; below that it is better to remain at rest.