So Sorry, My Island Now
S01E15 - Airdate: 1965-01-09
Runtime: 30 min
Gilligan is in the lagoon trying to catch food and see's what he thinks is a sea serpent. Later it turns out to be a Japanese world war II mini one man submarine who's pilot is not aware that World War II has been over for more than 20 years.
Episode details and rating from TVmaze.