Aquarium heaters will burn out if the water level falls below it's "minimum".
What I did to get around that, was take one of those novelty shot glasses, they hold like 6-1oz shots, and usually say something clever as you fill up to each line. Same diameter as a regular shot glass, just about 6-8 inches tall. Put the aquarium heater in the shot glass, and place the shotglass in the water dish. Obviously need to secure the heater/shotglass so it can't be tipped.
Another, easier solution, is heat tape! This stuff works wonders, just wrap around whatever needs warming and plug it in. Set it on a cheap timer to be on 15 min off 30 or whatever keeps the water a liquid. As a lil bonus, you can take the excess "tape" and wrap under the coop, for just a lil extra heat for the birds! It won't be enough to heat the coop to a huge degree, but it could just be enough to take the bite outta the coldness.
Heat tape is relatively cheap too, and expandable, as you can just run more tape, heat multiple water dishes, etc. You never know when chicken math will strike and you need more equiptment to house more birds.