Hi @mrpk, welcome to the forum 
There’s a 0.3 minimum range the device is capable of detecting (due to ringing vibrations from the pulse transmissions), but you’re past that, and it shouldn’t cause a distance estimate offset.
As I understand it the distance estimates are from the transducer location, which is roughly at the face of the sonar. The main ideas I can think of around measurement error here are
- there are objects within the sonar beam that are closer than the point you were measuring to visually
- the surface you are measuring has a gradual density change, or the sonar return signal is saturated, so the visual edge of the surface does not align with the results from the sonar’s density estimation algorithm
- this may be visible in the profile data
- there is some potentially relevant discussion in this thread
- the speed of sound of the water is substantially different from what you have configured
- this would cause a scaling error for all your measurements in that location