Last year we were fortunate to be able to fit a Lowrance StructureScan to Vixen, and after some faff we now get reliable reasonable quality images from it!  Thanks to the management at Stoney Cove, who allowed us to launch the boat and image the underwater features so that the equipment could be properly set up.  

Sonar images are affected by the speed of the boat during capture (variations in which can elongate the image and cause distortion).   The sea state and turbidity of the water (e.g. bubbles from divers or from the prop) can also affect results, so boat handling skills are an important part of survey activities.

As an example, here's the iconic M2 Submarine (near Weymouth).  This is a downscan-mode image.



This is the Black Hawk (Bow Section) and debris field:

 Black Hawk 21 rotated


Below, the German WW1 submarine UC70:



 The same wreck but this time Structure scan is overlaid over a conventional sonar image:

shot 91

Clearly showing the deck mounted gun in front of the remains of the fin (not the conning tower!)