Wow. You did well on that one.
They will gobble up duck weed and it will soon disapper. The same for hydrilla and the other "oxygenator" plants. Water hyacinth and water lettuce will last a little longer but they distroy it too.
Cattail has to be deeply rooted and if it is not a thick stand it will just fall over into the water. I don't think they will eat cattail. Once they become established, some of the other emergent plants like cyperus, papyrus, arrowhead (duck potato) or pickerel weed will stand up to ducks longer than most things. They don't eat water iris either but will soon stomp it down.
Having a separate pond or tubs that the ducks cannot access is good for growing duckweed and, to a lesser extent, the hyacinth and water lettuce. During the summer you can scoop a little bit out every day to feed the ducks and it will regrow.