I have 4. I am planning to block off part of the bowl so that they can't climb in.
I would think if you unplugged every night, you would also have to bring the bowl inside so that it doesn't refreeze. For the wiring to work so hard to thaw water every day, it would definitely burn out.
I work part time, so am not available to refresh water twice a day or more every day.
I kept hearing bad things about the heated fount I use for the chickens. It was given to me by the previous owner along with the chickens - it worked for him for the season and then me. That is two seasons that it has worked. I am hoping for a 3rd.
If the heated dog bowl doesn't work out, I'm out $15. I will then figure out another option. I might have another dish of water out in a rubber basin that will be blocked off too so that they cannot swim in it. I might have left some things out of my plan.
I will figure out what works for my set up - just like I did for my chickens. I was just throwing out my ideas.
I should have said unplug and dump any remaining water. I suspect with 4 ducks and a dog bowl there won't be much water left in there anyway. Blocking off part of the bowl is a great idea to keep them from climbing in and I think it will work well. I don't think the bowl is working any harder if the water in it is frozen or not. They just have a heating element that kicks on and off at a certain temperature and it's only running at barely above freezing, it's not heating the water to 80 degrees or anything like a fish tank heater. If the little water left in mine froze overnight I would just pour a pitcher of warm water into the bowl and it would melt out whatever ice was in there.