Shriveling is a defense mechanism against extreme cold. It can happen when you're exercising, or when your scared or stressed too. Your scrotum tightens also to prevent the temperature in the testes from getting too low.
You can try giving your penis a light massage and stretch, but the real remedy is to get out of the cold. You could pop a couple of VigRx pills before entering the water to maintain a "semi", but if the water's too cold, it may not help much.
