If you don't already have a heated waterer, there are other ways to deal with this.
--Don't worry about water while it's dark. It will freeze, but they wouldn't be drinking in the dark anyway.
--Give them fresh water as soon as it gets light in the morning. Cold or warm is fine, but not hot enough to burn their mouths.
--If possible, check after 2 hours or so. If the water hasn't frozen, they might be fine until evening. But if it is frozen, give them fresh water again and (if possible) come out in another few hours.
--Make sure they have unfrozen water in the last hour or so before dark, when they are trying to eat & drink before dark.
The most important times are first thing in the morning, and when they are eating before bed. They can survive the middle of the day with frozen water if they have to (like if you have to be away at work.) And if snow gets in their pen, they might eat some of it too. (Don't count on snow for all their water needs, but eating some snow will not hurt them and will provide a small amount of water.)
It can also help to give them a dish of wet chicken feed first thing in the morning, and again shortly before their bedtime. They can gobble it up quickly, and then you know they got a cropful of food AND water at least twice in the day. Just chicken feed + water and let it sit for a minute or two.
