I give some of them a smaller tub of water and have a 250 watt stock tank heater. The one that I use looks like cast aluminum, keeps the water thawed but doesn't get hot enough to burn their feet. (I just don't use the ones that look like an oven heating element because they get hot enough to melt plastic and I am sure would burn them) I use a small enough tub that I can pick it up and dump it daily. Keeps the water totally open even when it gets to -30F. But it does get a lot of ice build-up around it from the water that they splash out... The ducks that are in an area without electricity have access to the outside (where there is snow) and I just give them a small rubber tub of water. I take it outside of the pen, turn it over, and stomp on the bottom until the ice falls out. Then I take it back in and refill.