I do not know what environment you are living in. But thick more swampy-plants (cat-tail sorta/reeds/grass species) can help with keep the water clean; and are often sturdy enough to stay alive with ducks in it.
But it all depends on how big your pond is. and if you don't already have those. And if you like them
If a smaller pond; I think I saw someone over here that made a bio-filter in a loose bin, with a system that you had to pump it a bit daily by hand (just a bit of the water, not all ot it). But that was not a natural pond and had a faucet in the pond to clean it; since the ducks tend to bring a lot of dirt in to their ponds.
If you have a deep natural pond; there are bottom-eaters that do not show much/and or can't be eaten. But that also depends on where you live. We had luck with a big species of algae-eating shrimp (too big to eat).
But what mainly worked; plant plants plants. All over the sides in less-deep parts since the ducks tend to go to the open deep part. You can also make a ridge so they don't go in those shallow parts as fast. And their active washing/swimming sorta revolves the water around.
We don't let them swim now in the natural pond; because now is the time to plant plants and throw in some water-fleas to clean it. We do the same in September (both with a plant-residue and bottom cleaning before it). And they just have to do, unfortunately for them, with a kiddie pool for three weeks

But hey, noone can have anything all of the time; and it gives in the end a better pond for them