The owners of this pen say that they really like the lobed-shape of their pool because the ducks can swim in their own areas. Based on their other photos, I don' think they have a drain, so they have to lift the whole thing out when they need to clean it.
This is a guide about making a duck pond. Raising backyard fowl is becoming quite popular. If you choose to have ducks they will need a some type of pond. (they don't NEED a pond, but it sure would be neat to have one for them!