The Water Demons are the main antagonists of an episode of Are You Afraid of the Dark?, "The Tale of the Water Demons".
Two cousins deliver food to an old captain who lives by the lake. After seeing treasures from an old sunken boat being displayed in his home, they find out that he is being stalked by water demons (who are actually the drowned victims of a boat capsizing who want their jewelry back) whenever he falls asleep. When one of the boys secretly steals one of the captain's items, they start to be followed by the demons as well.
In the end, the brothers help the captain to throw back all the treasures into the water, appeasing the Water Demons so that they leave the captain and the brothers alone from then on.