I've never seen or heard of wild mallards nesting by salt water. All of the dabbling ducks I've seen in marinas in Spring and Summer have been domestics, probably released by owners who didn't want them any more.
From what I understand they posses all the qualities of a wild mallard haha. To clarify the water is brakish, with a rather low salt content.
In Chincoteague Va (not too very far from the OP), there are populations of both wild mallards and mallard/domestic crosses.