My chickens have sand, and I put a couple of ducks with them to see if I wanted sand for them too or something else.
Something else for sure! The ducks got 2.5 tons of pea gravel, which filled there 10x10 run 6 inches deep. The sand got so hard and compacted, fast. It's not like raking it once a day is enough, as soon as the add water and tamp it down with those flat duck feet, it becomes a hassle.
Gravel, hands down. I didn't even put a base of sand down, I dug drainage (away from the coop), filled that with gravel, laid down landscape fabric to keep the gravel from sinking, and then all that gravel right on top. Blast it with the hose for cleaning.
After having it for about 8 months now, the hosed off poo composts below the gravel level. Worms have taken up residence and the ducks look for them. When I hose, the worms come to the surface and the ducks eat them.
Once a month I use a garden sprayer in the evening and spray bleach into it. After the ducks have gone to bed. Then the next morning the ducks are let out into the yard and not their run, and I sniff the rocks until the bleach smell is gone from sitting in the sun (deactivates it). I started to notice an odor from the composting poo below when I used a gravel rake. Little bit of bleach into that wet mess under the gravel stopped it. With time to deactivate, it won't kill the ducks either. The hose sprayer mixes it, you can control how much you use.
The gravel also made it so that their pool has a higher elevation, just enough to be able to drain it with a hose into different parts of the yard. No more dumping it in the same area. The gravel also keeps them SO clean! The pool water lasts at least a full 24 hours... instead of just 2 hours when it's surrounded by dirt. With gravel, you only have to clean the pool when it gets poopy.