Question

What is happening in this diagram?

Answer 1

It looks like sonar. Sound waves are sent downward into the water, they reflect off something (the shipwreck), and bounce back up to the ship.

Answer 2

In your picture, is represented how a sonar works. Namely the sonar send a sound wave, then that sound wave hits an object which is upon the bottom of the sea, and subsequently, the same sound wave is reflected towards the boat from which it was generated. In so doing, that reflected sound wave, hits a receiver, which is located at the position labeled with C