As cold as it gets. I am from Eastern Europe but now staying in south of France for couple months. When I am home it can be as cold as couple degrees above 0, especially in winter on the countryside.
Now in the south it's not so cold which bothers me, but I use the coldest possible out of the principle, as to keep the principle is the key.
One of my favorite things is to sauna, then cold shower, then brief sauna again, and my nerves get totally reset such that I can go sit in a -20C chair outside and will not be cold for a surprisingly long time. It feels great.
If the water is not cold enough, it is disappointing.
Now in the south it's not so cold which bothers me, but I use the coldest possible out of the principle, as to keep the principle is the key.