I feed my ducklings either Purina Flock Raiser or Purina Meatbird, both of which are made with ducklings in mind and will meet their nutritional needs. The extra nice thing about Flock Raiser is that you can continue to feed it to ducks for their whole lives with no need to change food when they are older.
Ducklings can also be fed a regular chick starter if you cannot find a flock raiser type feed or a waterfowl feed. You will just want to supplement additional niacin, since chick feed isn't made with the nutritional needs of ducklings in mind and doesn't have enough niacin for them. Brewer's Yeast is an easy to find supplement that you can add to the feed that will make up for the lack of niacin in the feed.
While ducklings have no need for medicated feed, you can feed it to them if that's all you can find or if that's what you already have. There is a myth that ducklings cannot eat medicated feed. While that used to be true, back when feed was medicated with 'sulfa' drugs, feeds are no longer medicated with sulfa drugs. Amprolium, which is what most feeds are now medicated with, is totally safe for ducklings to eat. So while medicated feed is not necessary, if that's all you can get, you can safely feed it to them. I verified this with my avian vet to be sure.
For all my older juveniles and adults, I feed Flock Raiser or a chick starter/grower, which is what I feed all my birds. Both of these feeds are 20% protein, and I find my birds do well on it. I offer oyster shell free choice on the side for any laying birds that need it. They will take what they need, and that way ducks that aren't laying and drakes aren't taking in more calcium than they need, which can cause problems for them.
For treats, I give them dried mealworms, live feeder fish in their pool, and they get whatever kitchen scraps I'm tossing out for the chickens too and they take what they want from that.