AAAAAND Bob and the antitheists around here missed the point. Shocker.
@Review: I figured as much. It was a book that only worked because it was a book.
@Quirky: Bob, you do realize that every Tuesday the Escapist has a video in which you tell us about how your zany/relevant/weird/silly hobby or obsession is so awesome, right? Pot, kettle, et cetera.
Now then, Life of Pi. SPOILERS SPOILERS SPOILERS CLANG CLANG whatever.
Life of Pi was a book about the importance of making a choice, regardless of whether that choice would be the correct one in the end. Essentially, we all have a choice in how we choose that which is ultimately unknowable (a la the veracity of Pi's castaway story).
There is the side which is ultimately a little more uplifting. The side which makes you feel a little better about the coming dark in your final moments. The one that aids in the perception of a world that is just a little bit more manageable, a little bit more hopeful. You can choose to believe that not everything in the world unfolds in the most logical, practical way. That maybe things just aren't explainable sometimes. That maybe somewhere out there is something that is making sure everything turns out OK in the end. The downside being that anyone who disagrees is going to casually call you a dumbass on every internet board in existence, and also the possibility (probability, even) that you will one day be proven wrong.
Then there's the other side. The "right" side. Everything is explainable. Pi could never have survived that long on a boat with an actual tiger because, well, hungry tiger. There is no God because the universe as explained by the sciences has no need of one. The upside here is obvious: You're right. You'll always have that. No-one can prove you wrong because your policy is to believe that which has been proven correct. The downside is that sometimes being alone or sick or hurt or about to cease to be can be damn depressing. Sure, it often isn't, and being right is fun, but a lot of people like to think there's more out there. Also, the other choice has assholes who will call this group a bunch of idiots. Assholes are kind of everywhere.
So, make a choice. That's all the book is saying. Make a choice, one way or the other. Don't sit on the fence until one side "wins," because neither side ever will. Just make a choice, and live the happiest and most harmonious life you can regardless of which way you believe in the end. Because the choice doesn't matter, remember? We need to know how the ship sank, and Pi's story has no bearing on that at all. But if you never decide whether it was people or animals on that lifeboat, it'll be a hell of a lot better to figure out why the ship sank.
As for why Pi is multi-religious:
Most people who believe in God(s) don't fit completely into one religion. Hell, no independent thinker who believes does. We all realize that the Bible is the re-re-re-re-written re-translation of a bunch of hearsay and legends that had been running around as an oral history for quite some time by the time it was recorded. Thus, most believers tend to stay away from the conservative extremes of their given religion and migrate toward a more universal view. It's pretty much the only way to come up with satisfying answers for the major questions asked of spirituality. Pi is a Muslim, a Christian, and a Hindu because those three religions mesh together well. They, like most major religions, operate on the basic principle of "don't be a dick." It's pretty much exactly like Ethics. Anyone who studies ethics knows that very few people operate strictly under a single ethical ideal. This is because those ideals each have their own weaknesses, and living by one ideal exclusively exacerbates those weaknesses. Yet, despite not living under a specific ethical code, most people have a strong sense of what is ethical and what is not. In the same way, religious extremists amplify the weaknesses of their dogma. Most religious people are not extremists, yet we have a strong sense of what we believe and why we believe it.