As we've previously noted in 17 Facts You Might Not Know about Gilligan's Island, producer Sherwood Schwartz had to make many changes to the premise of the show to get it on the air. Studio executives were very skeptical that it could work. At one point, an executive known as "the Smiling Cobra" told Schwartz that he had to turn in the lyrics to a theme song for the show . . . the next morning! Schwartz was no song writer, but he realized that with a deadline like that, he'd have to do the work himself:
I has several friends who were songwriters, but who could I call at 8:00 p.m. to write a song by morning? I would have to explain the whole idea of the show and get someone to incorporate in the lyrics all the exposition I wanted in the song. No, that was hopeless.
Ignoring the fact that I was trying to do something that couldn’t possibly be done, I began to write the lyrics for Gilligan’s Island.
The result was this odd song. Unlike the song which eventually aired, you can't sing the words to "Amazing Grace" to its melody. Also, the Professor, Mary Ann, and Ginger have different professions. You can read more at Dangerous Minds (content warning: foul language).