I tried Niacin in the water ... you need to get just the right kind of niacin, and it needs to be "refreshed" at least once per day as it is a water soluble vitamin and therefore deteriorates quickly in water ... and you know how ducklings are with water ...
I think a more practical solution is adding a niacin supplement to their feed, and that can be done with either Brewer's Yeast or Nutritional Yeast. There is a brand of Brewer's Yeast which is promoted on various poultry forums ... it also contains garlic, which you may or may not want to be feeding to your birds.
However, Nutritional Yeast has a lot more niacin than Brewer's Yeast.
Here are links to the nutritional data for each type of yeast ...
Brewer's Yeast:
http://nutritiondata.self.com/facts/custom/1323569/2
Nutritional Yeast:
http://nutritiondata.self.com/facts/custom/1323565/2
Though the headline suggests a "custom" quantity, I've "tweaked" both of those to display the nutritional value of 1 ounce (28 grams) of each so it is easier to make a direct comparison.
You can see 28 grams of Brewer's Yeast has 9.3 mg of Niacin.
You can see 28 grams of Nutritional Yeast has 98 mg of Niacin.
So, Nutritional Yeast seems a more practical option to me. You can add less. It is what an animal nutritionist recommend I use ... sprinkled over their feed.
How much you would add depends on how much is already in the feed you are providing, but as niacin is water soluble, there is little worry about overdosing with the niacin ... I'm not so sure about the other nutrients in the yeast products, though.
Nutritional Yeast can be bought in the bulk bins of grocery stores ... I'm pretty sure Brewer's Yeast can be found there, too. Certainly both are available on the shelves, but that would likely cost more than the buying it in bulk.