Get a heated dog bowl. I am SO buying one for next year. Another option is an old round cooler filled with warm water at night. It usually lasts until morning in liquid form. That's what I did in January. But if you have electric, I would go for the heated dog bowl.
Here is mine...
Actually their hutch could be a lot smaller, but I made this out of an existing container and I never had ducks before, so what did I know?
The price was right....FREE