Duckweed and vallisneria (jungle val)
are two very inexpensive, fast-growing options. The vallisneria can be planted in the bottom of your pond and allowed to spread on its own. Duckweed grows on the surface of the pond as a floating plant, is high in protein, is easy to grow, and spreads quickly.
If there is already water in your pond and, like me, you have no intentions on going for a cold swim every time you need to add a plant to the deepest parts of your pond, you can tie each plant to a small rock, aim, and toss it in the general spot that you would like for it to grow in. Something you'll want to keep in mind is that your ducks do not care about whehter or not your plants are established before they start gobbling them up, so in order to prevent your garden ducks from eating your newly "planted" pond plants before they can get to spreading is to keep the water level high on your pond.
For those who have access to a source of water to fill up the pond, this, of course is easy. Otherwise, this is something you'll want to take care of several weeks before the arrival of your ducks, if possible.
Although you don't need to "add" common garden weeds like dandelions, ducks have also been known to snack on these too, adding more variety to their diets.
It has been noted by aquarium keepers and in my own experience with trying to get duckweed established in my own duck and fish ponds that jungle val (vallisneria) releases a chemical compound to weed 😬😬😬 out the competition. The competition being... duckweed. What this means is that you cannot grow both vallisneria and duckweed in a single pond unless you plan on flooding the pond on a regular basis (which removes some of the compond produced by the jungle val making it possible for the duckweed to survive).
I don't recommend this as it can be a lot of additional work, but, there is also the benefit to your plants of being regularly flooded with high nutrient duck or fish water, and as ducks will eat both, duckweed, and vallisneria, it might be worth the effort to pump out the duck water as liquid fertilizer for your plants, or to just flood the system every now and then.
Also be sure to check out: How To Prepare Your Edible Landscape to Welcome Indian Runner Duck(lings)