Just got back. Also saw it with no idea what it was about I just have a theater nearby that has a really big screen and does 3d movies awesome (which I love despite the fact most do not)...and like you I thought this movie was absolutely amazing. Extremely good tale, probably one of the stronger deliveries of "the message" it carries I have ever seen, and absolutely STUNNING visuals. Those scenes where the ocean lights up, and everything is like space above and below...beautiful. And the boat crash was truly terrifying (and that scene where he sees the boat from underwater was breathtaking). Anyway, loved the movie had no idea what to expect and this is by far the best picture I have seen this year. No contest.
I thought about that also...since everything else matches up pretty well. There's the obvious take on it that he did find some island for a while but took off b/c he had to get back to society and felt he would be devoured by island life living alone if he didn't keep trying to get rescued.
Also there's a little garden of eden type metaphor....but finding that island and the tooth...regardless of how extraordinary the rest of the film was up to that point...took it to the next level and really makes you doubt the truth of the story. One would say as a viewer...that scene provoked my (the viewers) rejection of the miraculous story sort of an expression of struggling to accept faith. Which sort of makes it more powerful I suppose when the end comes and you get the general message of the movie hammered home.