Hello, and welcome to the BYC community.
Do you have a picture of the eye?
In my experience, eye issues in waterfowl most frequently stem from trauma often due to excessive mating from males, but also after getting their eye poked in objects such as wire, or branches.
Lack of deep bathing water can encourage the onset of conjunctivitis which can create considerable swelling, and discharge which may lead one to believe the eye is absent.
How we care for the eye depends on how the eye actively looks, and the underlying cause, which is why a picture and close examination of the bird would be recommended.
If you can take her to a veterinarian experienced with birds, that would be a great route to take.
Otherwise, please post more information and pictures on the duck forums:
https://www.backyardchickens.com/forums/ducks.42/