How can I best adapt my body to hot weather, and cold water/weather?

I live in Arizona (phoenix) and have all my life. I am already adapted to the heat without a doubt; so mainly I am asking How can I "get use to" and be "comfortable" in cold weather (less than say 55*F) and chilly/cold water when submerged (aka a pool) while staying adapted to the hot weather as well -- that I am already use to?

Hopefully that makes sense.

Should I start taking increasingly cold showers, while running/being active (and properly hydrated) in the day time? The weather never gets cold here and I'd have to travel a ways for it to be noticeably chilly, so I want to focus on acclimating myself with cold water while retaining my heat endurance.

Thanks