Jaws - Sense people in sea

All at once he sensed another vibration in the water. It was on the surface, but it lacked the smooth pulsations of an amphibian. A seal - even if there had been any in waters as warm as this - would have been slinking with half the noise and sisturbance.
A human, perhaps a small one.
The graceful tail began to sweep again.

(...)

He increased the beat of his tail. He centered on the vibrations of the living thing he sensed close to the beach. He suddenly became aware that two more beings had entered the water near the surf line.

(...)

But now the original quarry, closest to him, stopped, turned back, and moved toward the two new sources of vibration.
For an instant he glided, as his brain evaluated this new data. Then his tail began to move again, dragging a vortex along the ebony surface above. He raced for the group as it merged.

***

Jaws: The Revenge, Part 2, Chapter 12