Gilligan and the Skipper building a raft at the Lagoon
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 Lafrance and her husband Ricardo Laughingwell, who formed the South Sea Film Productions. After they where married they sailed on there yatch to the island on there honeymoon to make a motion picture together. There Yatch somehow sunk, leaving there film, camera, and costumes for the castaways to find years later.
The lagoon as it appears in the credits in season 2.