What are some metaphors and similes in The Cay?

Metaphors are nonliteral comparisons used to describe something by comparing its qualities to something not normally comparable. They are among the most common tools in authors' toolboxes for adding poetry and variety to stories.
Similes are a specific type of metaphor that are constructed using "as" or "like." But many metaphors leave the comparison implicit.
Consider the following line, when Philip first sees Timothy on their liferaft:

He crawled over toward me. His face couldn't have been blacker, or his teeth whiter. They made an alabaster trench in his mouth, and his pink-purple lips peeled back over them like the meat of a conch shell.

The description of his teeth and mouth as an "alabaster trench" is an example of a metaphor. Alabaster is a very white mineral, and a trench refers to a long, narrow ditch. Neither of these literally apply to a wide smile, but they create a striking mental image of how wide and how white his smile was.
The description that his lips were "like" the meat of a conch, a type of sea snail, is a simile. Just as with the metaphor, the author is creating a vibrant visual image. But this one uses the word "like" to explicitly call out the comparison, so it counts as a simile.
Note that neither of these comparisons are nice. The choice of metaphor isn't purely visual; by having Phillip compare Timothy's face to a "trench" and his lips to a "conch," the author shows us some of the dehumanizing effects that racism have on Phillip—his first instinct upon being greeted by a helpful, friendly African is to think of him as being dirty and slimy.
A few paragraphs later, we see a similar pairing of metaphor and simile:

His voice was rich calypso, soft and musical, the words rubbing off like velvet.

The first half, stating his voice was calypso (a type of music), is a metaphor, and "like velvet" is a simile. And already we see Phillip warming up, using musical comparisons instead of unclean ones to describe his companion.