Muscovy Duck

AKA: The Turkey Duck.
Frequently found in public parks this lovely specimen is pretty much ugly as ugly can be.
They are larger ducks, males getting up to nearly three feet and females commonly around two feet. They're dark with white wing patches, and they have the bare red turkey face that just might make them distinctive enough to ID from, I dunno, 50 feet away. Muscovy have claws/nails on their dark feet and a wide tail that is reminiscent of a turkey with it's tail down.
It's native range is lower Texas through Mexico, but they adapt easily and can even handle temperatures down to 10* Fahrenheit. If you see them wild in a park up north, while they may be wild, they (or their parents) were likely brought in by people so as to have something other than mallards.
There are domesticated varieties of this duck but some of them are hard to tell the difference between wild, feral, and domestic. Domestic types are normally lighter colored and fatter, but if you're dealing with a duck that lives off people feeding it I'm not sure if you could tell if it was bred chunky or just over fed.

On a neat note though: Turkeys and Muscovy Ducks are the only two domesticated ducks that are native to America.