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 salt water 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.