Bread is bad for ducks because it lacks any nutritional value for them. It can be fattening and make it harder for them to fly and otherwise escape predators. It can also lead to other problems:
Overcrowding: where an easy food source is abundant, ducks and other waterfowl will lay more eggs and the pond or lake will become overcrowded. This makes it more difficult for birds to seek out healthier food sources and increases the likelihood of territorial aggression.
Pollution: when too much bread is offered to ducks, not all of it will be eaten. The soggy, uneaten bread is unsightly and rotting bread can create noxious odours as well as lead to greater algae growth that can clog natural waterways. This concentrates the pollution and can eventually eradicate fish and other life in the vicinity as well as being harmful to dogs.
Diseases: feeding ducks bread can increase the spread of diseases in two ways. First, a carbohydrate-rich diet leads to greater defecation, and bird faeces easily harbour bacteria responsible for numerous diseases, including avian botulism. Second, mouldy bread can cause aspergillosis, a fatal lung infection that can decimate entire flocks.
Pest attraction: Rotting supplies of food leftover from ducks will attract other unwelcome pests such as rats. These pests can also carry additional diseases that can be dangerous to humans.
Loss of natural behaviour: when ducks become accustomed to handouts, they lose their natural fear of humans and may become aggressive in order to get more food. Their loss of fear can also cause dangers, such as a willingness to cross busy roads in order to reach picnickers and other likely sources of food.
If you can entice the ducks out of the water and feed them on the ground, it is much better for the pond.