I use a hard plastic heated dog bed. It's supposed to have a fleece cover over it, but I removed the fleece cover and set the metal waterer directly onto it. (it is waterproof and has a heavy wire wrapped cord to prevent chewing). it does not overheat the water; the water stays thawed but does not get warm. It worked out perfectly all winter.
I plan on using a heated dog bowl (holds 5 quarts) from Tractor Supply. It is $20 and only runs when the temp drops. That's what I use in the barn for the barn cats to drink, and it works perfectly.