We are in Georgia so it is not *supposed* to snow here but last year it did and our kiddie pools would freeze over, not much ... what do you know the ducks would jump in anyway. They are built to walk on ice, it is really interesting if you look into it the way their feet are designed to walk on ice. I don't think they really care with all that fluff anyway.... water is water and ducks are ducks lol
EDIT: If you live somewhere where the whole thing would freeze I wouldn't bother filling it and keeping it thawed. Just make sure they can clean their bills and eyes in their drinking water.