I worm my ducks, you should work all of your poultry. When I worm, I do 2 things. 1) is Peprizine, or Wizine ( same product different brands) in the water. You can over use and be safe. Yes, its ok if the ducks bath in it. The second thing I do is get a tube of Ivomec horse wormer paste. I do this 2 times a year, early spring and late fall right after a freeze. I take the horse wormer and squeeze it all out in a line, then I cut it according to weight.. or I just put a dab on my finger, either way, you will get it figured out fast how much goes on your finger with out all the stress of math and cutting the paste. I normally make a few cuts to get a mental picture and go from there. Anyway....
I just catch them, open their mouth and wipe the paste on the upper part of the bill. My chickens, guinea ducks, everything gets wormed this way 2 times a year. I even do this with the dogs and rabbits. The Ivomec is helpful because it gets external as well as internal and the peprizine is just an easy in the water way to go that takes no time at all and is very cost effective.
If you don't worm, you will have issues. Worming is one of the cheapest and most important things you can do on a farm.