spoiler: whats with the writers?

Supposedly, they DID film the book's ending, but it got cut.

This film has long been one of the film industry's dirty little secrets. Leading up to its premier, there was a nightmarish struggle on set between the writers, director, and producer regarding how the movie was going to be shot and then marketed. It's pretty hard to come across "making of" information on the film because everyone involved got tight-lipped in the aftermath; but from what I've been able to glean, the movie was originally a lot closer to the book, and stuff ending up getting cut out in post-production by power hungry producers and distribution executives who wanted a different product. Before "Waterworld" traipsed along a few years later, this was actually one of the biggest flops in Hollywood history; since Disney bankrolled it, though, they had the cash to do some good PR work and sweep it under the rug.