The last boat I had was equipped with side scan. I had an absolute blast with it. When the fish weren't biting I would just toddle around looking at stuff to see what I could find. The older fish finder I had often would not show anything on the bottom that was shown on nautical charts. Then my uncle {captained the research vessel for the UMD} explained it...he said that whenever you see on a chart a little circle of dots and the letters "obst rep pa" {obstruction reported, position approximate} this is where someone reported to the USCG something in the water, but the USCG was unable to find it when they went out and "sounded" for it at that spot. As opposed to the little boat with the letters "wk" for wreck. Doesn't mean it's not there or at least nearby, or it seems like they must never take any of those obstruction marks off the charts, so it could be corroded away to nothing by now and there is nothing left of whatever it was to see. When I got the new side scan I was able to find some of the charted objects. Sometimes I could tell what they were, sometimes it was too far gone. A lot of fun never the less. Also confusing on the charts at first was the letters "PA", in capitals...this designates "private aid" which is a channel marker {usually just a stick} or a homemade buoy someone other than the CG has placed for whatever reason.
One other reason I found for stuff not there was local lore. The Chesapeake bay and her tributaries have many "rock piles" that are well known places to catch fish. I couldn't wait to go and check them out with my side scan...only to find that better than half such places had no rock pile or anything else there...but apparently fish. Sure enough, when I asked the old timers why they believed there was a rock pile or other obstruction they said, "well, there must be somethin', we always caught fish right there."