"He stopped and put his mouth to the lake and swallowed great gulps of it. He drank until his stomach was swollen. Where he was immediately sick and threw up most of the water" (Paulsen 43).
Obviously, you can tell that Brian is desperate enough for water that he'll even drink the mossy, infected, gross water. The water that Brian drank can represent many things but mostly life because in the beginning of the book the lake water is the only thing that keep him alive.