How does Martel immediately create tension at the beginning of Chapter 37?

At the start of Chapter 37, Martel's first sentence tells us about the sinking of the ship. Thus, we immediately know that Pi's challenge has begun..... and that something is moving about in the water near him.

The ship sank. It made a sound like a monstrous metallic burp. Things bubbled at the surface and then vanished. Everything was screaming: the sea, the wind, my heart. From the lifeboat I saw something in the water.