Here's a duck I saw near my parents house in Orange Park, FL. It was in a neighborhood pond, and while I was in town, I saw mallards, muscovys, and mottled ducks in area. The other thing is that the female this drake was with had the same white patch around its eye.
I know that mottled ducks are related to and commonly mate with mallards, but I would expect something much more mottled-like than this picture. Until I hear to the contrary, I'm going to assume it's some type of mallard-muscovy terminator duck!!!
Any waterfowl biologists out there?