Depth sounders work with ultrasonics. When the boat is moving faster, the most common cause for depth sounders and fish finders to fail is turbulence. This is because the ultrasound image is garbled by the bubbles generated on a fast-moving hull. If you own a power boat, planing is another potential cause of the problem. A planing hull that is out of the water will, depending on where the transducer is mounted, cause the sound waves to exit the boat into the air, rather than the water.
Transducers must be mounted in a location where they will experience minimal turbulence and always be in the water. While generally transducers can’t be moved, it is possible to disconnect a poorly installed transducer and install a second one in a more suitable location.