Pi muses on fear, which now totally overwhelms him. His crippling terror overwhelms his reason and saps the energy from his body. The adult Pi comments on how fear is the “only true opponent” of life, and so we must constantly work to overcome it.
As a narrator, the adult Pi can muse on the fear that almost killed his younger self. Richard Parker is also just trying to survive – fear is the most dangerous thing in the lifeboat.