Can wild ducks eat bread?

We recently joined a marina where tons of people sit on there boats and feed ducks and geese bread. There are easily hundreds or thousands of ducks and geese that frequent the marina waters. I told my neighbour that she should not be feeding the ducks bread. My neighbours response was why? What can I feed them? I was at a lose for words. I'm sure that I'll have this conversation with them again what should I say?