Why does my penis shrink when it is exposed to water? every time i shower when i go in the pool? even with a warm/hot shower it shrinks a lot.

Likely safety: In an era before clothing and other external protections, it is likely this was just nature's way of pulling your "important" part into closer protection of your body.There is a muscle on either side that actually draws your testicles up and back.The body also doesn't like extra heat loss through external structures.This is similar to why your fingers & toes get so cold in cold weather.

Penis shrink: Vasoconstriction. Nothing to worry about