Ask Mr. Smarty Plants

Mr. Smarty Plants really hates to admit being stumped, but this may be the one. We have lists of deer-resistant plants but not duck-resistant. When you say you "live by" ducks and swans, do you mean someone keeps a swan and duck farm next to you (if there is such a thing) or do you live along a waterway of some sort? We found a quote from a book, Encyclopedia of Water Garden Plants by Sue Speichert and Ann Lovejoy, that does mention ducks and swans savaging plants:

"Geese, swans and ducks, although they are fun to see visiting, love the very same plants you do, but for food and nest building. Geese and swans love grassy things; anything grassy-looking is fair game for geese and swans. Ducks tear up plants but seem to do their damage by nesting in the plants rather than really eating them. Harassment is the best we have been able to do for birds. Loud noises, sprays from a garden hose, dogs, picking up their nests-all give birds the message they are not welcome. We have had much success with a motion-operated sprinkler."

Then we found this list of California Native Plants that Attract Birds to your Garden from the San Luis Obispo Chapter of the California Native Plant Society. We scanned the list and found a few that listed attracting ducks, at least, so you would probably want to avoid those plants. 

Beyond that, about all we can suggest is either seeing if the owner of the duck and swan farm (if there is one) could contain his flocks a little better, or moving your gardens farther from the water.