Symbolism, Imagery, Allegory
The algae island might be the second weirdest part of the book. (Second, maybe, only to Pi's conversation with the blind Frenchman.) It's an island made entirely of seaweed, full of meerkats and freshwater ponds. It gets even stranger: dead fish rise to the surface of the ponds at night and disappear by morning.
Initially Pi thinks the island is a delusion:
I was getting used to my delusion. To make it last I refrained from putting a strain on it; when the lifeboat nudged the island, I did not move, only continued to dream. (2.92.9)
But Martel spends too long with the island for it to just be a delusion. Pi describes the island very precisely. It just doesn't have the hazy feel of delusion. It has edges. One possibility is that the island represents some type of comfortable faith. When Pi first steps onto it he says as much:
My foot sank into the clear water and met the rubbery resistance of something flexible but solid. I put more weight down. The illusion would not give. I put the full weight of my foot. Still I did not sink. Still I did not believe. (2.92.12)
Doesn't that sound like "Doubting" Thomas from the New Testament touching Christ's wounds in order to believe that he was resurrected from the dead? Or Saint Peter trying to walk on water after he sees Jesus do it?
But maybe the island doesn't represent the type of faith Martel thinks we should have. Because, of course, the algae turns out to be man-eating algae. It's an island that can consume you if you're not careful. Perhaps, if your faith is too easy and you no longer brave the stormy seas, then you're no longer experiencing real faith.
Notice too that Pi really tames Richard Parker on the island. He has him jumping through hoops. Literally. Richard Parker, like the ocean, is part of Pi's spiritual trial. What do you do when your spiritual test (a.k.a. Richard Parker) follows your every command? You leave:
By the time morning came, my grim decision was taken. I preferred to set off and perish in search of my own kind than to live a lonely half-life of physical comfort and spiritual death on this murderous island. (2.92.143)
Like all of Martel's symbols and allegories in Life of Pi, the island ends up being more elusive than one might think.