When Gilligan's Island debuted in 1964, CBS promoted the show-a comedy about seven people stranded on an uncharted Pacific island-as a ''farce.'' The critics agreed, adding the adjectives ''moronic,'' ''stupid,'' and ''preposterous.'' One disbelieving reviewer said it was ''one of the worst shows that ever hit the small screen. Just too inane for words.'' But not for America. ''I thought it was goofy,'' says Russell Johnson, now 69, who played the always-steady Professor. ''I had no idea when the show first went on the air that we'd have ratings. People loved the idea of being with us on the island.'' Yet Gilligan's low-rent humor never wore well at the Tiffany Network. CBS rejected the series when it was first pitched by producer Sherwood Schwartz (who went on to develop The Brady Bunch). Schwartz finally won out, thanks to fine-tuning and the showbiz pedigree of his cast. Bob Denver, who played the bumbling, inept Gilligan, had spent four years on the Dobie Gillis show. Jim Backus (Thurston Howell III) was best known as the voice of Mr. Magoo. Natalie Schafer (the vacuous Lovey Howell), Alan Hale Jr. (the Skipper), and Johnson were veteran performers, while Tina Louise (the voluptuous Ginger) was a rising actress. Only Dawn Wells (Mary Ann) was unknown. By Christmas 1965, Gilligan was America's favorite show. Nevertheless, says Johnson, ''I don't think (CBS was) too thrilled with us.'' The Professor, as usual, was correct. When CBS presented its fall 1967 schedule to chairman William Paley, he was shocked to see that Gunsmoke, his and wife Babe's favorite show, wasn't included. Obeying Paley's orders to save Gunsmoke, programmers bumped the still-popular Gilligan off the schedule. After three seasons and 98 episodes, the final show aired Sept. 4, 1967. The indirect result of the Paleys' whim was that Gilligan went on to become one of the great syndicated hits of the '70s, '80s, and '90s, spawning two Saturday-morning cartoon series, three NBC TV movies (the first, 1978's Rescue From Gilligan's Island, finally returned the castaways to civilization), and talk of a feature film. Denver, Wells, and Johnson occasionally reunite at Gilligan cruises throughout the country (Louise, the only other surviving member of the cast, was never a fan of the show and doesn't take part). ''It's absolutely amazing-it's crossed three generations,'' says Johnson. ''It's as alive today as it ever was.''