Actually, Rhimmer, that was the following movie they made, the Castaways on Gilligan's Island. It took place immediately after the events of the previous movie, and in fact Mr Howell then bought the island, built a resort on it, and made everyone partners. It then became a sort of landbound Love Boat scenario, with various stories about the visitors.
They made one other movie, the Harlem Globetrotters on Gilligan's Island, which is another level of junk TV Nirvana entirely.
I really shouldn't know so much... :-)

I remember watching an other movie with a diffrent ending where they find an old airplane and rebuild it to get off the island. They got the airplane to work and took off but soon lost power and had to land. Luckly a Coast Guard plane so there plane landing and came by to see if they needed help. Later they found out Mr. Howie owned the island all along.