https://youtu.be/JWfULMcFikI

Rolled oats seems to work pretty well. They tend to float and the ducks like them (and it's not bad for them).

My local swan goes nuts for peas but eats out of my hand and they're all bread spoiled

I managed to feed our swans easily some grass i throwed in the water.
The geese are hesitating often but sometimes eat grass. And the ducks only eat grass if either swans or geese already ate because they probably are too bread trained

But even if the greens go to waste they should not pollute the water compared to bread