Most of the book is focused on the theme of survival. The first third of the text has Pi narrating about his life in India. There isn't much "survival" happening there unless you count the government instability making life tougher for Pi and his family.
Once part two starts though, survival is Pi's number one goal. He has to survive the sinking of the ship. Then he has to survive for months in a lifeboat on his own. He has to hunt and scavenge for food, purify his own water, not die from exposure, not go insane, and not get eaten by a floating island. Oh, and did I mention that he has to do all of that with a hungry tiger in the lifeboat with him?