Tough question. Normally the climax of a book is super easy to nail down. That's not the case with Yann Martel's Life of Pi. I've actually read one analysis that listed the ship's sinking as the climax. I can see the reviewer's point. It must have been a harrowing experience, and it does basically completely wipe out Pi's previous life. In that case, it is a major turning point and could be considered a climax.
On the flip side, once Pi is in the life boat, just about every waking moment could be a climax point. Everything is life threatening. Everything is hard. Every decision that Pi makes could in fact lead to it being his last decision. By the way, if you like that kind of writing and plot, read Andy Weir's "The Martian." Back on track, listing a whole bunch of parts is not a good answer to your question either.
I will give two possible climax points. You can pick from there. Personally, I think it is the floating island of death and subsequent escape. That was some messed up stuff. I mean Pi found human teeth in a tree's fruit, which leads to the conclusion that the island itself eats people. He and Richard Parker then high tail it out of there. I've always thought that sequence was the most harrowing sequence of the book. That's why I think it is the climax.
I've had students support the idea that the climax is Pi's and Richard Parker's arrival in Mexico. Richard Parker runs off, and Pi gets rescued. I understand why people think that is the climax. Everything after that point is just falling action and resolution. Not much actually happens after their Mexico landing. I just feel that compared to the island of death, arriving in Mexico is enough of a let down to be included with falling action.