which details of rainsfords struggle to get to the island build suspense?

He's in the ocean and it's dark,

"He struggled up to the surface and tried to cry out, but the wash from the speeding yacht slapped him in the face and the salt water in his open mouth made him gag and strangle."

He hears the screaming of a mysterious animal in the distance and swims towards it,

Rainsford heard a sound. It came out of the darkness, a high screaming sound, the sound of an animal in an extremity of anguish and terror.

"He did not recognize the animal that made the sound; he did not try to; with fresh vitality he swam toward the sound. He heard it again; then it was cut short by another noise, crisp, staccato."