I have a special needs pekin -- deformed bill after a raccoon attack when he was a little fluffie. He came to me as his "forever home" once his wounds were healed 2 + years ago. He is a small boy -- probably because he was malnourished as a duckling while healing from his wounds. But one difficulty he has is in preening. He spends as much time as my other ducks preening; his preen gland is functional, but he can't reach his under carriage to keep clean and preen properly, He rarely gets in the kiddie pool but when he does he has a good splashy bath. I put him in the kiddie pool once a week to ensure he bathes, and give his underneath a good rub while he is there.
Now he has "gone off his legs" for almost 2 weeks and he is needing special care and treatment. I am putting him in my bath to exercise his legs three times a day. He doesn't like being in there. When I get him out he is water laden and I have to stand him on the edge to "drain" his feathers, before taking him on to a towel on my lap. He isn't a cuddly duck, he's a back yard pet who is curious and comes up when I have treats but has never wanted picking up or cuddling. When I am rubbing his underneath with a towel, he is as good as gold, and rests his head on my shoulder. I don't need to restrain him, So I think he knows he needs to be dried off [or he will get a chill from the air conditioning in the house]. I put him back outside in a dog pen during the day but after the evening swim, it is really important to dry him off.
I have been doing this for nearly 2 weeks now and I am hoping it will stimulate more waterproofing, but I am not yet seeing that. I am thinking that he and other disabled ducks have special needs, and disabled ducks just aren't going to waterproof adequately. But it doesn't matter as long as they don't go into water. I am continuing with my frequent swimming regime to try and get him moving again, but once he is up and moving about normally, I don't see the point in making him go in in the kiddie pool when he doesn't want to. I will bath him about weekly to keep his underneath clean -- and more often if I see his feathers are fouled -- but I don't think its important that a duck swims.
You might think about whether its neceesary for your ducks to swim. They don't need waterproofing if they aren't made to go in the water. You might be stressing yourself unnecessarily if there is no reason for your ducks to swim. It's likely part of the special needs of a disabled duck that they don't waterproof adequately and just need help keeping their underneath clean.
As it happens my other ducks are muscovy drakes and they are stinkers!!! One voluntarily goes in water about once every 2 weeks; one goes in far less often, and one drake only went in voluntarily once last year to my knowledge. I hose those boys down when they get to ripe!!
So although we tend to think that ducks like and need to swim in water, that might not always be the case!!! [In contrast, my son's ducks are all water babies: they would be unhappy not having access to a kiddie pool.]