[dbbolton]

The peripheral vasoconstriction is part of a reflex known as the mammalian dive reflex in which your heart actually slows down, rather than work harder, as soon as your face touches cold water. Muscle cramping is a direct result in the decrease in cardiac output.

https://en.wikipedia.org/wiki/Mammalian_diving_reflex

[shiggerino]

Another thing to note about mammalian diving reflex is that the nasal cavity is especially susceptible to triggering it. Which is why it's important to hold ones nose when jumping in the water to abandon ship, regardless of how silly it may look. And at this point you don't need a life jacket, you need an immersion suit.