Because the path can be very long, local fluctuations are averaged out. We can also use measured fluctuations in the signal caused by internal waves, turbulence and other variables to learn more about these smaller scale ocean properties. Shine a flashlight on a rainy night and you can see the light scattered back from rain drops. Sonars can transmit a narrow beam of sound into the ocean in a short pulse. The echo can be detected.
Bubbles are especially good targets because they ring like a bell.
Density steps and layers can be imaged with sonar so as to delineate internal waves and other small scale structure. An aircraft or police siren seems to emit a higher tone as it approaches and lower tone as it departs. This is the well known Doppler effect, also used by bats and dolphins to track prey. With a back scatter sonar we can measure flow speed from the frequency shift of echoes from bubbles or plankton - a valuable tool for ocean studies. Breaking waves, rain, snapping shrimp, moving sediment, marine mammals, sea-ice and countless other sources contribute to the naturally occurring sound field.