How does Theodore Taylor use imagery in The Cay?

Examples of Imagery:

"Father was smiling and very gay but when the Hato's whistle blasted three times, meaning it was time to go, he said good-by to us between clenched teeth. I clung to him for a long time."

"Something slapped up against my leg, and I thought it was Timothy. I knew how to swim, but didn't know which way to go. So I was treading water. Then I heard Timothy's frightened roar, 'Sharks,' and he was thrashing about near me."