Water, Water Everywhere - S1.E14

Talk about a painful irony: though surrounded by water, the Castaways suddenly discover that their supply of fresh drinking water is running out. When they pool all available drinking water together and decide to ration it, they discover that they will each receive only a few drops per day. Of course, most of the castaways sneak water from the community supply when Gilligan is standing watch, but return it when the Skipper catches them. Just when all seems lost, Gilligan saves the day!