I use a nipple system in the duck pen. This keeps the pen very dry. They only use this when they are penned up at night. During the day they have access to many open deep water containers. I have been doing this for a while with no adverse effects. I use the bright orange nipples that can be tightened by a bolt and inserted into the bottom of a suspended 5 gallon bucket.
I have a bucket with chicken nipples in my duck pen (as well as a larger swimming pond) and even the week old ducklings use it (with the aid of a brick to climb up to reach it--as it is set at adult height!)