"Pain and discomfort in the legs of niacin deficient birds is quite common so letting them relive the weight off their legs may help significantly during treatment, also birds that sit around most of the day aren't able to able to practice hygiene and are going to be more likely to end up with other problems like wet feather, foamy eye, bumblefoot, or external parasites, letting them in the water may help to prevent most of these problems for occurring.
When doing water therapy, given that the duck's leg are most likely weak, they should be watched the whole time they are in the water to ensure they don't drown."
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/