Hmm, lots of food for thought.
Yes, that should be a good size for them. I would place the swim area in the lowest part of the pen. The shallow channel would run along the low side, in your case you may want the channel to be a V shape, as the low spot is a corner. Or a Y, actually, with the base of the Y leading to the mulched mulberry (and perhaps comfrey under the trees, perhaps to an area where you can grow some amaranth or other seeds for the ducks).
I recently read that perennial rye is not necessarily a good thing for the ducks to be on, by the way. I do not think it is common, but I read an article about rye making some waterfowl's skin (legs) photosensitive, causing necrosis. Yuck.
Ducks will trample grass. I have a nice base in our day pen - it took a little time for it to balance out, but I add a little chopped straw and that mixes in with the duck manure and the ducks and I stir it occasionally, and it holds a little moisture (which the maple tree likes) and attracts worms, that the ducks forage for. The maple also seems to appreciate the manure.
A friend of mine lets some shrubs grow right up in the middle of the pen - she prunes the shrub so it's about a foot short of the top of the pen (she is very good about security and has a very heavy netting all across the pen). I have coated woven wire across the top of our day pen.
I have seen some beautiful duck pools here on BYC. Trying to recall which thread.
Oh, before you put the ducks on the area - I am wondering if you might need to spread anything like lime to be sure there is no residual parasite in the soil….laugh if you need to, if that is never a consideration where you live. In some places, there are disease organisms that can persist in the soil. Ducks often have their faces in the soil, looking for goodies, so it is something to think about.
