The type of water you're using the sonar in affects its operation to a large degree. Sound waves travel easily in a clear fresh water environment, such as most inland lakes.
In saltwater however, sound is absorbed and reflected by suspended material in the water. Higher frequencies are most susceptible to this scattering of sound waves and can't penetrate salt water nearly as well as lower frequencies. Part of the problem with salt water is that it's a very dynamic environment - the oceans of the world. Wind and currents constantly mix the water. Wave action creates and mixes air bubbles into the water near the surface which scatters the sonar signal. Micro-organisms, such as algae and plankton, scatter and absorb the sonar signal. Minerals and salts suspended in the water do the same thing. Freshwater also has wind, currents, and micro-organisms living in it that affect the sonar's signal - but not as severely as salt water.
Mud, sand, and vegetation on the bottom absorb and scatter the sonar signal, reducing the strength of the return echo. Rock, shale, coral and other hard objects reflect the sonar signal easily. You can see the difference on your sonar's screen. A soft bottom, such as mud, shows as a thin line across the screen. A hard bottom, such as rock, shows as a wide line on the sonar's screen.
You can compare sonar to using a flashlight in a dark room. Moving the light around the room, it's easily reflected from white walls, and bright, hard objects. Moving the light onto a darkly carpeted floor returns less light because the dark color of the carpet absorbs the light, and the rough texture scatters it, returning less light to your eyes. Adding smoke to the room (children, don't try this at home!), you'll see even less. The smoke is equivalent to salt water's effect on the sonar signal.