Clean by virtue of having been washed in water(adjective)
Washed or swept with water especially waves of the sea(adjective)
WordNet. Princeton University. 2010.
Use "water-washed" in a sentence
"It was a rough ride, over water-washed boulders down the Dyea Valley; but he roused only when the wagon hit the highest places."
"At times, he seems a kind of ancient mariner in this water-washed realm, and in one scene, which features a series of lit and then extinguished matches, he becomes a fire-bringing Vulcan on a strikingly minuscule scale."
"He came back with great leaps across the jam, one huge hand filled with round, water-washed pebbles."