If the water is frozen, the ducks won't come in. You can either go hunt somewhere else where their isn't ice -- on a river with moving water (and in fact on such days the presence of ice on competing wetland areas will concentrate the ducks to the remaining wetland areas without ice, such as a river) -- or else you can break up the ice on your body of water to make a hole for the ducks to land in. The ducks, because they have good eyes, will see the ice and avoid coming in otherwise. If the ice is more than 1/4" thick, you might as well give it up at that spot. Also, get the ice out of the way, because if your decoys are sitting in a field of ice, they aren't going to look natural and the ducks won't come in.