Like many birder/nature photographers, one of my earliest exposures to the beauty of birds was ducks in the park. It was almost a family ritual when I was growing up. As soon as winter released the upper Midwest from its icy grip, we'd make a run to the local day-old bread store (remember when there was such a thing?) and then have an outing to the neighborhood park pond. It took less than a week for the mostly Mallards and "funny ducks" to become habituated and begin excitedly quacking and swimming toward the pavilion any time a car stopped.
There are two reasons feeding ducks in the park makes wildlife managers cringe. It's bad for people and it's bad for the ducks. If you're people, concentrations of ducks, and particularly geese, can quickly foul high usage park areas with their droppings, and feces in the water can contaminate ponds with algae blooms and bacteria such as E. Coli which humans really don't want to experience.
If you're the ducks, your digestive track is not equipped for the sugars and salts of bakery goods and marshmallows. Waterfowl are evolved to live on aquatic vegetation and insects. They need the nutrients, especially protein and calcium, from these natural foods to thrive, reproduce, and produce healthy young. Additionally, mold on uneaten unnatural foods can infect the ducks and even area songbirds, and assemblages of large numbers around popular feeding areas increase the chances for the spread of avian diseases.
Predators and other uninformed human duck feeders aren't dummies either. Both will quickly notice where concentrations of wild birds occur, and both will take advantage to the further detriment of the ducks you're trying to love and enjoy. Part of the joy of birding should be the opportunity to observe birds doing what they are evolved to do in nature. With the possible exception of endangered species, they don't need our help and won't benefit by our meddling in that evolution.
One morning last winter while birding at my "local patch" which comes replete with a pond and some unfortunately habituated local ducks, I observed a lady drive up in an SUV and pull a full size shopping cart from the rear storage area. It was completely filled with breads, pastries, cracked corn, overripe bananas, and a watermelon(!). I am not making this up.
The woman proceeded to spend over an hour preparing and distributing this "bounty," and the list of species attracted was impressive--funny ducks, Mallards, Northern Pintails, Northern Shovelers, Ring-necked Ducks, starlings, grackles, Cactus Wrens, even overwintering Yellow-rumped Warblers. There was an accipiter lurking around the area too.
Fed ducks are dead ducks, and there are a lot of ways for them to die. You don't need them eating out of your hand to enjoy or photograph them.