On the top of the duck's tail area, they have a gland that produces oil. This is the preen gland or uropygial gland. They will repeatedly go to this gland and get the oil all over their bill and head, then smear it around on their feathers while they are grooming themselves. This is why you...
With my first ducks I waited a while before I introduced them to water because of the oil issues mentioned, but one of them was constipated for the first 3 or 4 days and had weaker legs even with niacin supplements and electrolytes so she was "floated" in a warm bowl of water under tight...