How do I get my ducks to have healthier feathers?

The ducks feathers are waterproofed by oil from a gland near their tails . . . they rub it into the feathers when preening.   It sounds like they have been upset by the move into a new place, but it sounds like you have all the right things for them, so I would imagine once they settle down that they will start preening more and you should see more waterproofing  . . .

Do you know how old they are? I would offer them a vitamin booster to their water to help out a bit. I also offer lots of treats to newcomers so they know they are welcome and cared for. The ducks go gaga over meal worms (pet stores carry them) and salad in their pool water. Make sure they are getting fresh clean water to bathe in frequently so they have an opportunity to preen really well - they oil up their feathers after bathing (oil glad above the tail)- you can watch them do it frequently during the day. If none of this helps - there could be something wrong but if they are new to you...do you know what kind of care they received before you?

This is the time of year when many from last years hatch (and older) are molting. Make sure you keep doing what you're doing, and give them access to some fresh plant matter as well. By fall they should be good as new.  

Ducks can get worms, thought they are not as prone to them as some other animals. In Storey's Guide, Holderread says he never has to worm his ducks, but always worms new ducks during their quarantine. It might be worth taking a fecal sample to a vet, or even just worming to be safe since you know they came from a wormy environment. In reading his description of symptoms of a heavy worm load, it doesn't sound as thought the poorly oiled feathers are really a symptom of that however, hopefully those will be fixed with a moult and plenty of time to preen.

The poor feathers are probably just a lack of access to bathing water. If you give them bathing water that will help. Water stimulates preening and gets that oil gland working. But they probably just need a good moult too. Over time, feathers get worn out and don't 'work' any more. As you'd know, feathers have tiny hooks that hold the feather strands together (the tiny hooks that are lacking in Silkie chickens, meaning they just have fluff) and these hooks are really important for waterproofing. They need to be in good condition for the feather to have a nice smooth waterproof surface. When ducks preen they are distributing oil (which keeps the little hooks nice and flexible) but they are also straightening up and re-zipping all the little hooks. Over time, the hooks get worn out and the feathers can't repel water as easily, even though the oil gland is still functioning. So then it's time to moult.

With water access and a moult your birds will be good as new. Sounds like they have won the lottery, coming to live at your place. I'm sure they are going to love living with you.

Personally, I would worm your new birds. It's extremely likely that they have worms, given your description of the other animals where they came from. You don't want them introducing a whole new zoo of worm eggs into your property. Worm them with something that covers tapeworm, to be on the safe side, and then worm them again 10 days later. That's what I would do :)

Good food and nice fresh greens will do the rest :)

Excellent advice from everyone! Looks like some yummy salad (my ducks personally adored any tender leafy salad green especially dandelion greens for some reason chopped up and tossed into their pool.), good food, lots of fresh pool water, a good worming and a moult and they are going to be gorgeous again!

For show birds, we give them Manna Pro Poultry Conditioner. It helps with their feather condition.

I think one thing that should be mentioned is Muscovy as a breed do not have the water proofness for lack of a better description as other ducks. It could be conditions, feed.. general health etc, playing a role basing on your info coming into play too..

I feed mine a flock raiser by Purina and they free range... i have to dash right now but i should show you my one duck she is done her molt and just shines she is super white so i know my feed program is working!