Ducks do release more moisture than chickens. They are more cold hardy too on average(not bantams ducks though). I would experiment with both options of vents. They don’t like vents or drafts at their level but above them should be fine. How large is your coop? Is it one big thing or is it a box(elevated with a ramp or not) and with  an attached run? I wouldn’t keep water in a small coop as they are going to spill a big portion of it wouldn’t be able to find nice dry areas. I assume you would let your ducks out of the coop in the morning? If you do this later in the day I would definitely put a water in their coop, they will want to drink when they wake up and start moving around.