Storey's Guide recommends, for ducklings showing signs of deficiency, 100 to 150 mg of niacin per gallon of water.
From this point, though, it gets fuzzy. How much water to ducklings actually drink? I don't know, it will depend on the size and age of the duckling, the weather, activity level, etc.
I do think that more of what's in the food gets ingested.
islandfeed dot com writes that 2 to 3 cups of brewer's yeast per ten pounds of feed would be their recommendation for supplementing chick starter for ducklings.
Also, I think I have read that thiamine is also needed for leg health (among other things). So, do you have poultry vitamins-electrolytes-probiotics? I think giving them some in their water once a week would be a good idea in any case.
Swim time - supervised closely - is a good exercise that takes weight off the leg and improves circulation, so I would be sure the ducklings get time in the water daily. It doesn't take long for ducklings to poop out, so be careful.