The ratings for "Rescue From Gilligan's Island" (italics mine) were such that NBC, starved for hits to begin with, commissioned a sequel from creator-executive producer Sherwood Schwartz. While "Rescue" was independently produced through Schwartz's Redwood Productions, it seemed apparent he couldn't do it alone a 2nd time, so he hitched Redwood's wagon to Universal for "The Castaways on Gilligan's Island", which premiered in May 1979.
This is actually 2 films in 1. The first half repeats the rescue effort from the first film. After discovering a lost plane on the island (which wasn't there in the series), the gang try to return home on their own, but the plane breaks down while attempting to retrieve Gilligan (Bob Denver). They're soon rescued by the Navy, and brought back to the mainland.
However, Thurston Howell III (Jim Backus) decides to build a resort on the island as a tribute to the time spent there.
That sets up the second half, intended as a pilot for an anthology series in the vein of Aaron Spelling's Love Boat over on ABC. Publicly, Howell owns the resort, though his wife, Lovey (Natalie Schaefer) and the rest of the gang are "silent partners". Tom Bosley (Happy Days) and Marcia Wallace (ex-The Bob Newhart Show) are among the guests.