From what I can tell from my guides, these are wild Muscovy Ducks. They were seen on the far banks near the Inn at Chachalaca Bend, but would never come to our side of the shoreline. Based on the red knobs and darker color, I would say that these aren't domestics - or at least, are only domesticated birds of wild stock (and not the generic domestic stock that has a much more ... well, domestic, appearance).
Feel free to correct me if you know better. Filling out my lifelist from our trip to the Lower Rio Grande Valley.