Liquid water, might want a heated waterer.
Or just carry water several times a day.
Fresh water when the sun comes up or the light goes on, because they'll be thirsty when they wake up. Fresh water again (if needed) sometime before they go to sleep, so they don't go to sleep thirsty.
If the water freezes quickly, they will get thirsty, and then they won't eat. But you can add water to chicken feed to make a warm mash, and they'll gobble that down (food and water and tastes like a treat, all in one simple dish!)
It is possible to serve a warm mash morning and evening, and have that be enough food for the day, if you get the quantities right (although I would leave dry feed available, and fill the waterer at least twice a day, anyway. That way they can get a bit more if they need it.)