I have a big pond for my ducks...and the 10 of them made it such a poopy mess that they were getting dirty and sick from it....and the bobcat kept hunting them there. The pond is great when it's raining and a stream fills it and the extra water flows down stream. But, in the summer, the water level gets so low and the pond so gross that it actually smells of
poop. So, even a pond has a downside if you can't drain and refill it.
At another time, I had a big cement pond for them to swim in. It had a drain at the bottom, so I could water the garden with the water. But the drain always got clogged and it took SO MUCH time--and water--to drain and refill it. We tried to filter and airate it, but it wasn't working out.
Here's a picture of the big cement pond. It's four feet deep at it's deepest:
So, I switched back to just using 1.5x2.5 foot, 6 inch trays. They're random plastic "drawers" that my husband found in the recycling at work. A big oil pan would also work. These are big enough that the ducks can take a bath. and so the water gets pretty muddy/poopy pretty fast. So, I tip over this water at least once/day. Sometimes I rotate where the tray of water is, usually by a fruit tree, so I can just dump out the water and fertilize/water a tree at the same time. On hot days like today, I leave a hose trickling cold water into their biggest tray to keep it cool and full for them.
Here's one of their old trays. This one broke, and their new ones are slightly larger and deeper, but it gives you a general idea:
I also use a pail for them to get drinks out of, which they have a harder getting dirty. It's deep enough that they can stick their heads in, and tall enough that they can't get in it, while also not being so narrow that they tip it over. I also change this water once/day. One word of caution, DO NOT use a pail where you have ducklings...even big ducklings. I have had at least two ducklings get in and drown during the night. I also had a large duck who had wet feather (from when life was too crazy and I wasn't changing their water every day), and she somehow got in it and then died of hypothermia during the night.
Also, different ducks make different amounts of messes in their ponds. My first flock of ducks really didn't mess up their water much...then I got three ducks from another lady's farm, and these ducks all LOVED to get mud in their bills and rinse it in their pond/tray. And, of course, they taught all my other ducks this. My water went from getting dirty once every 2 days, to being mud in just one day.