My Pekins drink all the time day and night. I sometimes think it is just a mannerism or habit. However, I just rescued 2 abandoned pekin drakes in a dire state: filthy, lost weight and very dehydrated. It took a full 48 hours before they were fully hydrated -- and I knew they were fully hydrated as they started passing liquid poops. I had gone out for the afternoon and could not believe the mess when I got home -- albeit on a tarp, but wow!!! The boys are still eating as if they haven't seen food for a month but are now drinking normally for pekins, ie frequently. My muscovy do not drink as much as the pekins and their poop is not watery in the morning as they haven't bothered to eat and drink overnight. But they pass more liquid and often watery pool during the day when they have been in the kiddie pool drinking.
I think your ducklings are likely to have normal pekin poop if they are active, eating, drinking and running around. My son's pekins are very different from his rooster, who passes lumps of poop that are strong and have to be diluted with the hose to not damage the plants.