All I get is "---" readings on my depth sounder in certain places. I've noticed that these places have very soft/muddy bottoms and/or very dirty/silty water.

You may be experiencing the diffusion of the sonar signal in many directions resulting in signal weakening through Refraction or Backscatter, primarily due to the material properties of the areas .

Backscatter: The deflection of acoustic signals in a scattering process through an angle greater than
90 degrees. Backscatter is the term commonly used to describe the return of energy from the
seabed to the receiver in an active sonar.

Refraction: The change of direction of a sound beam when passing obliquely from one medium into another, where its wave velocity is different. Refraction is a type of ray bending that will affect acoustic returns for proper sonar imaging. This occurs when sonar pulses encounter thermal zones, silt, salinity changes. Under these conditions the beam can be refracted sharply to the seafloor severely limiting range.

If you are experiencing one of these conditions,  you may have to discontinue use.

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