This past winter was actually my first winter with ducks so I can only guess at the difficulty of a really cold winter with a week or so below freezing. I had purchased a heated water dish in case it got really cold but never had to use it. Their kiddie pool did freeze over for a couple weeks, but ducks don't need water to swim in. They only need water deep enough to fully submerge their heads in order to clean their bills and such.
That being said, they are happier with water to swim in. They can clean their feathers far more efficiently with a pool or small pond.
Now ducks and water is a messy subject because ducks will foul a water bowl in a matter of minutes if not seconds, so you get used to changing water at least daily. Much like coons, ducks will "clean" their food and dirt they pick up around the bowl.
Managing the water and feed area has been the biggest challenge so far in keeping ducks. Fencing was easy, housing was easy, and even free ranging has been easy.
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