The term "journey" operates on a number of different levels in this rich and profound text. There is of course the journey that Pi himself takes as he tries to desperately reach any form of land, but then there is also the more metaphorical journey he makes as he voyages into greater maturity and realisation about his place in the world. What shows Pi's maturity more than anything else is how he has to come to terms with his new life and the brutal realities of being completely out of control over his food and water supplies. He is very aware how precious food and water is, and he is also fully cognisant of the fact that he has no control over the continued supply of either of these necessities. Note how he expresses this in the following quote:
When rough weather abates, and it becomes clear that you have survived the sky's attack and the sea's treachery, your jubilation is tempered by the rage that so much fresh water should fall directly into the sea and by the worry that it is the last rain you will ever see, that you will die of thirst before the next drop falls.
Every single survival, whether it be from a storm or soemthing else, is constantly tempered with the fear of running out of food and water. In this case, Pi talks of how surviving a storm creates "tempered... jubilation" in him, because of how much drinking water he has been forced to witness going into the sea. The fear of dying of thirst before it rains again shows just how much Pi has grown up from the pampered, privileged young boy from India. His maturity is therefore expressed in his struggle for survival and how he copes with the recognition of his own mortality.