Regarding the above post, it's much easier to just add Brewer's yeast to your feed than try to force niacin down your duck's throat multiple times a day. You should just add brewer's yeast from the beginning and then you won't have a problem. @Amiga has lots of good info on niacin deficiency and how to treat it. Swimming is not necessary to correct it.
Swimming - when monitored constantly with ducklings can be beneficial. They aren't fully feathered and can get too tired, or chilled - keep the temperature about brooder temperature. At least one scientific paper reported that ducklings who have a chance to get into warm water start preening sooner than ducklings that do not. And without B vitamins, swimming doesn't - as far as I know - help with the leg and other problems from B vitamin deficiency.
I think someone could do their PhD on duck nutrition. I know there are experts, but sometimes I feel there are some gaps, and maybe it has to do with genetic drift, maybe it has to do with the quality of what we are able to grow in depleted soils, I don't know. Back to the topic of B vitamins.
Here are some thoughts I wrote down that summarize what I know and feel about B vitamin supplementation.
I would go with brewer's yeast. Forgive me for using capital letters, but for anyone else who may casually look over this thread, NOT baking yeast, NOT winemaking yeast. Brewer's yeast is a nutritional supplement and it provides niacin and a few other vitamins.
Generally, folks who supplement with brewer’s yeast use a tablespoon per cup of food.
Ducklings need about three times the niacin chicks do. And some ducklings get leg problems or seizures if they don't have enough niacin.
If you cannot find the brewer's yeast, or if you happen to have (here come the capital letters again, please forgive me) PLAIN niacin - NOT no-flush, NOT timed release, then dissolve 150 mg niacin per gallon of water for the duckling’s drinking water.
Another approach is to use B Complex capsules, and use the B3 - niacin - levels as a guide. Again, 150 mg B3 per gallon of drinking water.
Keep the little one on the supplements for about 8 weeks.
Sometimes it only takes a few days to see improvement.
You can also see how it went for Qwackers and Bentley from these threads.
https://www.backyardchickens.com/t/857143/treating-wry-neck-in-crested-duckling-edited-title
https://www.backyardchickens.com/t/761308/peking-duckling-with-crooked-neck-help