They need the water/pool water to help them keep clean and preen their feathers. Mine look disgusting if they don't have a little pool thing to get into a couple times a week. they now currently have a couple rubber pans with water and half of a plastic barrel in their pen for water.
suprsingly enough they just hop in the barrel, splash a few seconds and get out. or they just hang out on the edge of it and drink.
Really do need water for bathing and a clean bowl or two for drinking. Even then, they mess it up pretty fast. I fill a small pool and 3 large rubber feed pans with water every day, they are yucky in no time. Then they get a bowl of clean water before bedtime, which is gross in the am.
You can raise them with drinking water only but it is a duck and in my opinion if you choose to own water fowl then you have chosen to accept the responsibilities that come with them. I feel ducks need water to swim in to clean themselves and be generally happy ducks. That don't mean you have to buy a backhoe and dig a lake in your yard LOL but at least a little kiddie pool.
To add............ducks cannot chew and need water to swallow and digest their food. Also need enough water to keep their nostrils and eyes clean and healthy.