Like a frightened turtle! It also is a natural protection from toothy predators like pike and musky.
When I was in medical school we watched a video in physiology class demonstrating the effects of jumping into hot water, tepid water, and ice cold water. The thing I remember was that without equivocation the subjects blood pressure jumped up to an astonishing degree (stroke levels)by jumping into icy cold water. There was massive vasoconstriction peripherally, shunting blood to vital central organs.