The problem with sonar is, it's a two-dimensional representation of a (usually) three-dimensional object - reading a sonar display is an art form, one born out of frustration and long hours on the water staring at green blips the size of fingernails and learning to differentiate between discarded tires and rocks and things of interest and value. Equally and relatedly problematic is the fact that various things interfere with getting clear or reliable readings of the water - drowned forests, abandoned vehicles, baby carriages - it's amazing the things that we have the capacity to completely ignore once they're underwater.
In the field of underwater sonography, a distracter is anything that appears on a sonar display that will get in the way of a sonar operator finding what it is that they're looking for.