I use lukewarm water because it doesn't freeze as quickly. I have a placemat to put the bowl on so it is not on the cold ground and change the water out every morning and night to keep it from freezing.
This seems like a nice heated water bowl. I have not tried it since my bunny lives indoors but it has good reviews. It may be a bit large though. At about $20 I think it would be a good deal. Heated water bowl