I think the biggest mistake people make is turning on the fish symbols. Those will show any reflection as a fish. I think the arches are much more accurate but even then not 100%. Just remember you have to be traveling at least 3mph to see an arch. Other wise they will look like a long line. Personally, I don't bother look for fish. I want to see the structure. But there again, if you see a hurdle down there and a buch of arches around it, chances are they are fish. Lowrance has an on line guide that is helpful in learning their systems.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va