Ducks (geese, too) poop...a lot...and where ever they happen to be. Doesn't bother me all that much...wifey hates it.
Not sure about tame waterfowl, but have always understood that the wild variety carry with them the risk of introducing parasites in your fish, as they are part of the life-cycle of some of these critters.
I've got lots of wild geese and (probably due to the RES), so far at least, no sign of those nasty internal worms or "pepper" spots.
