The cookie tin water heater works really well for keeping waterers unfrozen. We use those for all of our waterers, except our turkeys who get a heated dog dish. I don't use the heated dog dish for a couple reasons with chickens (altho it could be done, I don't want any risks). I feel it has an opportunity for accidents (drowning banties, fire due to open heat element underneath the base, and frozen wattles that dip inside). The tins are very easy to make yourself. If you don't have the skills, order some from someone who does. They make your life really easy and you don't have to worry about changing waterers out or adding warm water to melt them.
Regardless of what type of heater base you choose, please use hollywood block or bricks to keep it away from bedding which could catch fire. And careful not to put it too close to chickens.
If your plan is to alternate wateres once they freeze, don't fill them. They will crack, expand and so on. Only fill half full or less in frigid temps and plan where you will put it to thaw once you exchange, as there is a chance of leaks.
Lots of good info on this post and a great opportunity for others to decide what will work best for them and to look at some new ideas.
Jody