I found it is a lot easier not to keep water out at night since the birds are asleep and not drinking and water will freeze solid over night. Years ago, our days were a lot colder and at first I was hauling up warm water every few hours to replace what had frozen. This got tiresome, so I purchased heated dog water dishes. However over the past 2 or 3 years, I have only turned them on once or twice during the winter season, our days just aren't staying as cold. You can also purchase heated chicken waterers, either on line, or possibly TSC has them.
All this said, my coop is directly on the ground in contact with the earth and the floor tends to hold in heat. So when I fill waterers first thing in the morning with warm water, rarely do they freeze during the day anymore. Hope this helps and good luck!