It had been two years since the last Gilligan's Island TV-movie had aired on NBC. Unfortunately, series creator Sherwood Schwartz was running out of ideas, such that he had to borrow a plot concept from a Saturday morning cartoon from the 70's not connected with his franchise.
"The Harlem Globetrotters on Gilligan's Island" (italics mine, of course) wound up being the final chapter for the castaways, in more ways than one.
For starters, Jim Backus (Thurston Howell III) had been in ill health, and limited to a cameo at the end of the movie. To fill the void, David Ruprecht was brought in as Thurston Howell IV, this despite the fact that in the canon of the series, the Howells had no children to continue the bloodline. Schwartz was left with no choice but to ret-con that detail. Ruprecht didn't quite go out of his way to imitate Backus' inflections and vocal mannerisms, and did a servicable job, such that, had they tried another film after Backus' passing, Ruprecht would've fit right in.
Secondly, Judith Baldwin departed after 2 outings as Ginger Grant, replaced by Constance Forslund, whom I believe was on one of NBC's daytime soaps at the time.
Finally, the plot-lifting I was referring to involved the Globetrotters.
In both of their animated series, the last of which aired on NBC, coincidentally enough, the 'Trotters had to solve their mysteries by playing a game of basketball---this device was also used in their appearances with Scooby-Doo---which was the lamest of devices. In another coincidence, Scatman Crothers (ex-Chico & The Man), who worked on both 'Trotter cartoons (1970, 1979), played the team's coach this time.
You might say that Martin Landau & Barbara Bain (ex-Space: 1999, Mission: Impossible) were playing against type as the villains of the piece, as they wanted the island for some sort of magic metal or some such. The title card is the closest we're going to get to any actual footage of the movie. There's a trailer, but the YouTube poster used that to dub his comments over the theme song.
Long time Los Angeles Lakers announcers Chick Hearn & Stu Nahan were brought in to call the game, if ya will.
There is a commercial available, but it is not solely for the movie. There is a scene with Forslund and Dawn Wells (Mary Ann) where a robot breaks into their hut during an exercise class. Just the sight of Wells in leotards & tights is worth the price of admission alone. Team Mary Ann, I'm sure, had a field night.
Sadly, the movie fell victim to the law of diminishing returns. Schwartz would fail that same year with The Brady Brides, which brought him back under the Paramount umbrella. His production company, Redwood, produced the movie, along with the sitcom, Harper Valley PTA, for Universal. Long story short, while the movies air from time to time, fans are more interested in the original series, and this was the weakest of the three movies.