Chicken waterers work, as long as ducks also have something to dunk their heads in.
Whether Pekin make weight at 12 weeks depends upon the quality of your forage and whether or not you are also providing a supplemental protein source. Mine, butchered at 8 weeks, were actively foraging, but also eating a good quality 20% protein feed. Just grass will not grow you good ducks. Ducks are bug eaters, not grass eaters.
They will eat grass, but it is not the ideal diet for them. When foraging, they are after bugs with greenery just to fill in.
If you want to raise meat with no input except for grass, I suggest that you raise geese.
Whether Pekin make weight at 12 weeks depends upon the quality of your forage and whether or not you are also providing a supplemental protein source. Mine, butchered at 8 weeks, were actively foraging, but also eating a good quality 20% protein feed. Just grass will not grow you good ducks. Ducks are bug eaters, not grass eaters.
They will eat grass, but it is not the ideal diet for them. When foraging, they are after bugs with greenery just to fill in.
If you want to raise meat with no input except for grass, I suggest that you raise geese.