Why was the film different from the novel?

It wasn't all that different, in my opinion. Yes, there were small changes (A) the introduction of Catherine's family, B) Theresa was an assistant rather than a columnist, C) Deanna was eliminated, D) Garrett didn't teach diving anymore, E) Garrett died rescuing someone instead of simply being caught in a storm), but other than that, the story followed the novel fairly closely. The major theme was the same, most of the major characters were the same, the story arc was essentially the same, and the ending was the same.

The reasons for those changes have to do with the fact that films are a different medium than novels, and what works in a novel won't necessarily work on film. I'll go through those changes and give the reasons, just so you'll understand the give and take in such a process. A) Catherine's family was introduced to create conflict. In the novel, Garrett's conflict could be explored with introspection, but that's impossible to do on film, since we can't simply shoot a scene of Garrett "thinking." We had to "see the conflict," -- hence the introduction of Catherine's family. In both cases, there was conflict; it was just done in a different way. B) Theresa was demoted in the film because it was necessary to have her "complete a journey" i.e. learn something from all she went through. In the beginning, she was an assistant, as her relationship with Garrett blossomed, she began writing for the newspaper. By the end, she had her own office. As in the novel, she "grew." C) Deanna was eliminated because Theresa's job had changed. Her character was replaced with someone younger, but essentially the same; funny, sharp and someone who cared about Theresa. D) Garrett didn't teach diving because of time constraints -- it was either teach diving or restore sailboats. We chose the latter because it seemed more original and interesting. E) Garrett rescued someone because it was important to show that he hadn't gone out into the storm to commit suicide. In the book that was clear, but had someone not read the novel and just seen the movie, it wouldn't have been apparent, since he was an experienced sailor. Hence, to make it plain that Garrett was ready to move on, he died actively trying to assist others.