So ducks need floor space since they don't perch, well not often
They will use next boxes on the floor level. You can use a building like a chicken coop, however make sure the floor has a waterproof covering, like linoleum, and I'd suggest making a wire covered area for the waterer to drain to the outside... https://www.backyardchickens.com/forum/viewtopic.php?id=143772 ...see nettie's pics...mine are in a chicken coop right now, but this summer I will be retrofitting the floor to be like hers
You just won't believe how wet the bedding will get otherwise...in my other coop where I can't cut the floor I will continue using cat pans with wire covers as stands for the waterers...this way I can dump out the tray when I fill the waterer.
Ok so but for outside, during the day, ducks need to be able to submerge their nostrils to keep them clear...they will need a kiddie pool or at minimum a deep bowl.
Try searching duck coops or duck pens on here and see all of the cool ideas!