I would suggest either tying oxygenators to stones, or planting in baskets and placing quite deep- so that they can get established before they are within reach of ducks beaks and feet.
Lots of marginal plants.
I wonder if you could rig up a row of canes along your shore with string between or mesh to deter takeoff and landing.
No fish, if you can bear it.
A supersoaker water gun!

How big is it?

I know frogs and toads don't seem to mind me floating in the pond (been known to do this with rubber rings in order to clear excess pond weed in middle of pond--to hoots of laughter from family but the amphibians seem nonplussed). So maybe they wouldn't mind something floating on occassion whereas a duck would? Large floating crocodile perhaps? I have heard a floating rubber snake discourages Herons, but I would think frogs would be senstive to this shape.

Have found the critters do love water starwort, even though it does mean you don't get to see them as much. This summer we have had up to 7 frogs at a time poking their heads up above it in the warm weather. Not an exciting plant, but they like it.

I guess it will also be what is outside the pond--rocks and logpiles and such. We have a logpile covered in ivy, and some rockeries which seem to act as amphibian apartment houses. Also covered ways to get to the pond using ground covering plants.

Love to see how it goes!

Chip