While sound moves at a much faster speed in the water than in air, the distance that sound waves travel is primarily dependent upon ocean temperature and pressure. While pressure continues to increase as ocean depth increases, the temperature of the ocean only decreases up to a certain point, after which it remains relatively stable. These factors have a curious effect on how (and how far) sound waves travel.
Imagine a whale is swimming through the ocean and calls out to its pod. The whale produces sound waves that move like ripples in the water. As the whale’s sound waves travel through the water, their speed decreases with increasing depth (as the temperature drops), causing the sound waves to refract downward. Once the sound waves reach the bottom of what is known as the thermocline layer, the speed of sound reaches its minimum. The thermocline is a region characterized by rapid change in temperature and pressure which occurs at different depths around the world. Below the thermocline "layer," the temperature remains constant, but pressure continues to increase. This causes the speed of sound to increase and makes the sound waves refract upward.
The area in the ocean where sound waves refract up and down is known as the "sound channel." The channeling of sound waves allows sound to travel thousands of miles without the signal losing considerable energy. In fact, hydrophones, or underwater microphones, if placed at the proper depth, can pick up whale songs and manmade noises from many kilometers away.