Both Chara and Southern Naiad are used by Ducks (as is widgeongrass). I don't know which may be better for Gadwalls but I think at your location it might be easier to establish Naiad. Naiad is much more flexible than Chara in terms of water chemistry. Given this, Naiad will probably produce a greater quantity of food for ducks ... but it may not necessarily be more preferred by ducks or the particular species that you trying to attract.
I would just mention that without fish, it may turn out that crayfish will get established in high density and prevent a good stand from developing.
Bump*****
Call
these guys. They sell widgeon grass and might be able to tell you the difference between the plants in terms of attractiveness to ducks and ease of establishment and other requirements.