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.
Seconded. I get mine here; worked wonders on my super tippy duckling, but I gave it to all of them daily for the first six months or so.