Camping in the winter makes everything harder, quite frankly. And, one of those instances happens to be your water freezing over during the night. You do have a few options at your disposal to try to prevent this, though.
For starters, if you are using a thermos, store it inside your sleeping bag overnight. If you do not want to do this, store it upside down in a snowbank. While this seems ludicrous, the snow will act as an insulator and hopefully prevent the top half from freezing.