Doc, you mentioned "But that doesn't mean you can only make movies of things people already know about. ... there was no long book series on the Titanic before that movie was filmed."
Historical events are different than adaptations of books, comic books and TV shows. The sinking of the Titanic is a well-known disaster. There were almost a dozen movies (and many books, stage plays, radio broadcasts and TV episodes) that came before the 1997 James Cameron film. The first movie came out one month after the ship sank.