The density of its water varies with depth. The salinity, temperature, and pressure all change. And they change the density. Finally, the density of water determines how sound travels through it. Those density variations do a strange thing to the spread of sound in the ocean.
Sounds go out in all directions in air.
But sound that originates at a particular ocean depth spreads only outward. Sound stays in a plane, if it has a certain density pattern. Sound also travels five times faster in water than in air. And it doesn't attenuate as fast. So they'll lower a loudspeaker near an island in the South Indian Ocean.