A Duck Like Bird

Do you know how many duck like birds are out there? Ever once in a while I find one that I can't seem to name.
Every week I stop by the marsh in Edgewood to look for the swan. There were about twenty duck like birds in the marsh the other day. I tried to look these guys up in my bird book. But I could not really identify what kind of ducks these were.
I went back the next day to get more pictures and they were gone.
This marsh seems to be a good place for migrating birds to stop over. I am always amazed when I see a new using the marsh. I will keep stopping by hoping to catch them on film.
But in the meantime if anyone knows tell me what kind of ducks these are.