I know this is the 'Slug Thread' and not the 'Why You Should Own Ducks Thread' but why would it need to be constantly wet for you to own ducks?
I had 6 ducks in a lovely suburban back yard. The only water available was a kiddie wading pool. They thought it was their own private lake. Although sometimes they would knock on the kitchen door and ask me to make it rain...actually they wanted me to spray water from the garden hose. Ducks are very good pets; they work cheap; they eat bugs, insects and weeds...okay, sometimes they get it wrong and eat the good plants, but ...
Okay, back to the slug fest.