Ok - scratch that, you guys have convinced me that I don't want any ducks.
We used to have Canadian geese but I finally convinced them to leave and they did make a mess. I guess I just thought a smaller duck like a mallard wouldn't really be a problem.
When we were considering putting in a pond, the state wildlife biologist recommended putting in an island for waterfowl to nest on. That greatly reduces the chances of predation from coyotes, foxes, raccoons, etc...
Moss - I put a small floating structure in the center. Only had it there for two years. Had to take it out every fall - winter ice would make a mess of it. It was a PITA also - lake is 80 feet deep down the center. Don't think they liked it that much. When the wind blew, it bobbed around and probably scared them.
Ditto this. I am trying to find a way to add an island to my pond!