Life of Pi was pretty amazing. Some reasons? Well, I'd agree that it's pretty unrealistic that this kid survives for all that time with a tiger living on the same boat, but remember something: all literature is symbolic, whether the author intended it or not. This whole scenario may just have been designed to prove that you can survive anything if you have the will to live.
One could also argue that the fact that this boy lands on a flesh-eating island made of carnivorous algae and discovers his predessor's teeth in one of the trees is rather far fetched, but that would be looking at it from science's standpoint. That could also be considered unrealistic, but this entire thing could be a metaphor for the way you live: living in the ocean vs. living in a pool. The pool is safe, realiable, and undamaged by the harshness of the sea, yet you miss out on the most enjoyable things in life, such as love and joy. Being Xeno the stoic gets you nowhere. The ocean, while unrealizable, allows you access to all your feelings; the ocean is a symbol of life. Pi lived in the pool before, but notice how it takes on the form of the carnivorous island once he's been at sea a while. It all means something, and you can say I'm on crack, but you know, I might be right.
Anyway, it was an amazing book, I read it in my lit. class last year and enjoyed it a great deal.