Question

Why swimming or survival floating will hasten the onset of hypothermia in cold water?

Answer 1

Hypothermia is the rapid loss of body temperature as a result of the exposure to the colder environment. Therefore it occurs due to a faster loss of heat in the body more than what the body could produce.
So the person is more likely to be diagnosed with hypothermia more often due to such imbalance.

Answer 2

Swimming or the survival floating will then expedite the onset of hypothermia in a cold water as one can be in the water for too long in order to stay warm. The body heat will then unable to warm the water around as the fresh water will then rush in to replace it.