The lagoon was in Studio City, CA
Studio City is in the valley north of Hollywood, an hour from the ocean. Back in the 60s, the area was studios and a few scattered homes. There was traffic not far from the lot where they filmed the lagoon scenes for Gilligan. This would prove problematic when trucks passed while filming lagoon scenes. Years after the show, in the ‘90s, CBS drained and filled it to make another parking lot.