I use a small "chick waterer" with a 1 quart plastic jar placed inside the dog bowl. This is just about perfect for 2 days of drinking water for my 4 chickens.
The dog bowl heater is only 60 watts, so I am not worried about the plastic in the dog bowl getting too hot. The chick waterer - I have no idea if the plastic is bad to warm up. I think it is all polyethelene (either #1 or #2), which is what the dog bowl is made out of.
I like the chick waterer system vs. just putting water in the dog bowl because:
I have two chick waterers, and so I can just swap the empty one out with a clean and filled one from inside.
The waterer inside the bowl doesn't get stuff kicked into it. Especially in my location - on a narrow 8-10" wide shelf with basically just space for the chickens to walk over and drink.
Hopefully less wattle-dunking, as mentioned by previous posters.
Since I clean the chick waterers, I haven't cleaned the dog bowl itself in the whole month I've had it.