That's great. Just to be clear I wouldn't criticise you at all for saving the duck. I'm sure I would've done the same thing.I take him out everyday, once is the morning once in the evening. He's not in a cage or anything just some big enclosure. I made it as big as I could.
We go find some bugs and worms for him to eat
The problems arise from folks who want to raise cute little ducklings and then dump them in parks or a place like you are describing like college campuses. . And lake areas where people feed them bread and such and the numbers of birds become so concentrated in those areas that it increases the likelihood of disease, malnutrition and depletes the area of natural resources for other native species of established flocks. .not to mention the nuisance factor for human residents with all of the poo they create.

And although it may not be common for ducks to kill each other I wouldn't say it is out of the realm of possibilities where over crowding is an issue and you would have multiple drakes engaging in gang raping the ducks and or fighting over food.
Not trying to discourage you at all for saving a duck. . just explaining why you have gotten some of replies here.
I have no idea if this is a reputable news source for your area but I saw a few articles about muscovy ducks in Florida that were heartbreaking. I had no idea it was such an issue there.
https://www.google.com/amp/www.sun-...l-pines-muscovy-ducks-20150604-story,amp.html