I read a book about being mentally tough and started taking cold showers. Years later, I dont take cold showers anymore, but getting into and staying in cold water is easy.
>Get in the water, full body submerged, all at once
>Get out of the water
You only felt serious pain for about 1-2 seconds, when you are out of the water, your no longer experiencing the pain at any significant levels.
>Go back in the water
Now that you have felt the extreme cold, the water wont feel as cold as before.
Enjoy shocking your friends when you swim in 40 degree water.
Note: It still isnt comfy and your bits will go numb, but you can easily survive 5-20 minutes in various cold water temps.