People have been fascinated by dolphin sounds for millennia. It appears that dolphins use these 'signature whistles' like we use names. You often hear a loud whistle from a nearby dolphin, followed by a similar sounding whistle from another dolphin. Sort of like a sound 'handshake' or greeting.
Echolocation works like 'radar' and is used by dolphins to find food - like schooling fish.
The dolphin makes a 'click' which travels through the water, bounces off an object like a fish, and then hears the echo. These dolphin sounds are well within the hearing range of people. It is reported that cetaceans have a large portion of their brains devoted to auditory senses. Therefore they may be able to convert sound into an acoustic image in a section of their brains which allows them to 'see' in the darkness of the ocean, or in the murky waters of river deltas. There are many 'noise' sources in the ocean that could act to 'illuminate' objects with sound that cetaceans detect.