I don't like Dirty Ducks so mine definitely bath at every opportunity I have to fill the pool. Keeps feathers clean and oiled.Technically as long as you provide water to dip their heads in, they don't need a pool. But, if you are going to continue to use the pool @chickens really suggestions will get you through the winter. Or go with one of those heavy duty plastic troughs with a birdbath heater as suggested.
My ducks aren't dirty. I took their pool away last week. I provide them with plenty of fresh water though. I will let them have the pool today when I go outside to work in the yard. I just don't have the time every day to empty and fill a pool so that it doesn't freeze into a solid mass and don't have the resources to hook up a heat source. My ducks will be fine. They haven't complained yet.I don't like Dirty Ducks so mine definitely bath at every opportunity I have to fill the pool. Keeps feathers clean and oiled.