I like the propane heater, but I don't trust the plug in ones. I have heard of two horse deaths using a plug in, and that's two too many for me.
EVERYBODY that lives in a northern climate has this problem, including me.
I use two methods for their water. March-November, I use a big stock tank, right now it's 60 gallons, but the previous one was 110 gallons. (That one sprung a leak so it's my baby chick brooder in the barn.)
During the winter, I use rubber buckets in the stalls, and I have both a 20 gallon round rubber waterer and a 30 gallon round rubber waterer. They don't hold much, and you can throw them, step on them, etc. to remove ice. And, YES, I haul water in buckets. Don't you love your horses enough to do this?!?!? **inflicting guilt**
It's not that much work, just part of the winter chores.
If you want to spend some money, there are non-electric highly insulated water tanks and stall water holders that will keep it liquid to about 0 degrees F.