I've done it and they eat it much like chicks, but with much, much more head slinging of FF.....they prefer it pretty dry and they have to have a lot of water, which then makes the water really ick because they are rinsing off the feed into it. I tried nipple buckets with the ducklings but they weren't getting enough water that way...at all...even had a few die of what I considered was dehydration from lack of enough water due to the nipple bucket.
I'd venture to say that ducks would do much better with fermented whole grains than they would actual processed feed. Later on, as adults, they did very well on the fermented layer mash where the grains were bigger.
If I were going to do it all over again I'd mix their mix much dryer even than I had done. The dirty water seems to be a fact of life with ducklings, so just make the receptacle deep enough for them to swim in as well so all the feed particles sink to the bottom and they have the best of both worlds...water and swimming opportunities.