The Lagoon is a small bay in the island which is connected to the ocean. The Lagoon had a waterfall being emptied into it, people sometimes swim in it which could be an indication that it is freshwater. Since the Lagoon's water is often calm, most of the island visitors are seen arriving or leaving by means of the Lagoon.
The Lagoon was also the site of the sunken ship of Fifi Le France and her husband Ricardo Laughingwell, who formed the South Sea Film Productions. After they were married they sailed on their yacht to the island on their honeymoon to make a motion picture together. Their yacht somehow sunk, leaving their film, camera, and costumes for the castaways to find years later. It is unknown if they were rescued or died on the island. (Episode - Castaways Pictures Presents )
The lagoon was also a source of food for the castaways on the island, for they caught fish, as well as lobsters in the lagoon. It is also the scene where several objects wash in from the sea such as a military attaché case, a crate of radioactive vegetables, a box of plastic explosives and other objects. Visitors also tend to land here, such as Boris Balingkoff, Jackson Farrell, Duke Williams, Erika Tiffany Smith and a Japanese Soldier among others. The Professor once placed a stick in the lagoon with markers to keep track of the water level on the isle. In one episode Gilligan put his lobster traps on that stick, and was placing it in deeper water to catch larger lobsters, which led the Professor and the others to believe the island was sinking. (Episode - Quick Before It Sinks). The Castaways have also dug up items here like a tiki statue, pirate chest and a rare gem that seemed to grant three wishes. The digging of a lobster trap seemed to cause a section of the bank to float out into the lagoon (Episode - Three to Get Ready).
The ending of the second and third seasons show the Castaways still waiting to rescue beside the lagoon. A "Mad TV" episode shows seven skeletons still beside the lagoon... still waiting rescue!