I bought a waterer that plugs in and has a thermostat built in so it only comes on if temps drop below freezing.I run it off an extension cord, pat 2 winters have been super easy because of this.I believe the brand is farm innovators?
It just depends really. If you have power or your coop is close enough to your house you could use a extinson cord for one of those base heaters theres a link somewhere on here how to make one for just a few bucks. I just have 2 waterers i keep one in my house and one outside for the chickens so when it freezes i switch it out for the water in then house. Then let the frozen one thaw out.