Dolphins' brains have areas for acoustical imaging 10 times the size of humans, and have areas for visual imaging about one-tenth the size of humans. In effect, the dolphin "sees" by use of sonar, sending out sound waves and interpreting the echo from the objects hit by the sound. While vision stops at the surface of the object being seen, sonar penetrates materials. A dolphin not only "sees" objects with sonar, it can tell something about the internal structure of what it is encountering.
When a cetacean surfaces and opens his sound-producing mechanisms to the air, most of the energy travels into the atmosphere. If we remember that the sound released into the air on hitting the air/water interface is mostly reflected, we can see that airborne communication is not of much use to the cetaceans in communicating with others under water. Under water with closed cavities, they can communicate over astonishing distances on the order of six miles for the bottle-nosed dolphin and the order of five hundred miles for the finback whale. This long-distance transmission is due to the increased efficiency of transmission of sound waves in the dense medium of the water.