I've raised ducks in the past and they are the absolute worst about waste. I never enjoyed raising them or geese for that matter. But my personality is the type to jump in and deal with any problems as they come. Speaking of problems I see a few mainly-
*Ducks are omnivores which means they might see your fish or/and plants as lunch
*How would you keep their waste in the system to fertilize
*and the need for an extra feed for the ducks
Personally I will admit that I am biased against ducks. I would much rather raise chickens, but to each his (or her) own. And if you do end up keeping ducks please keep us informed, I would be very interested in the results. All that aside there is something aesthetically pleasing to having a duck in a pond.
I think just the risk of transmitting salmonella would keep them out of any area that I was raising vegetables to eat, since it's carried in their waste.