I've found that unless you have about 8-10' of relief, or the structure you're looking for is really big, it's nearly impossible to locate on sonar without running a lengthy pattern and paying *really* close attention to the sonar screen.
To see the object, you have to cruise directly over it at a speed which will show you the difference between the normal bottom and the top of the structure. Harder to do than it sounds.
When diving, I go by the GPS (with WAAS corrections, usually) for known sites, and use the sonar for finding new structure, or things like the edges of pinnacles and such (i.e. an anchoring aid.)
Hope this helps;