The Cay is told retrospectively from our protagonist Phillip's point of view. The narrative is written in the first person, which means that Phillip is telling the story himself. He's a "central" narrator because he's telling the story about himself – he's the star in his own story. We get lots of insight into what the young Phillip is thinking and feeling, which allows us to see more clearly the personal transformation he undergoes.
On the downside, we don't really get to see what other characters, like Timothy, are thinking; we are dependent upon Phillip for all of the information we get. While the first-person perspective gives us a rich view of Phillip's inner life, we are left wondering about other characters' points of view.
Perspective Adventure: Try rewriting a scene from The Cay from Timothy's point of view. How would it be different?