There always seems to be a big mystery surrounding fish finders aka sonar units, some seem to think they will catch them more fish while others think they don't help at all but yet have one on board because it wouldn't be a fishing boat without one!  Both statements while correct need some clarification from my way of thinking, the first, they will catch more fish if they are in fact used properly to locate fish, but then the rest is up to the fish whether they want to bite or not.  The second point, I agree that a fishing boat "should" have a sounder of some sort, to not only mark fish but to mark drop offs and shoals, underwater structure etc.. Even those die hard fly fishing gurus are equipping their belly boats and pontoon boats with sounders now, and it is helping them catch fish!

I'm going to keep this as simple as possible for all us laymen types when it comes to electronics, the Readers Digest version as it were.  For starters, the make of the unit can/might/may/could blah blah make a difference in performance, I always say don't go with the cheapest and you don't need the most expensive, you figure out where that leaves you!  Most sounders come with more features than you'll ever use, so use the KISS system when using it.  My biggest pet peeve for guys wondering how I can mark those fish on the bottom and see them come up to the line, keep the sounder on Manual and not Automatic and keep the Fish Finder feature off!!  That's pretty much it!  When the sounder is on Auto with the Fish thingy on its picking up fish farts, clumps of weeds, air bubbles, sticks and all the other assorted "stuff" that floats or sinks in a body of water.  Next, turn the SENS to max, the Greyline to 16% or so, Surface Clarity to Lo, ASP off and your ready to mark fish!  Oh, another thing, don't expect your markings on the screen to emulate those in the "demo" version of the sounder, ya, some may look just like that but most will look like, well, I'll show you.  No, you can't necessarily tell the size of the fish from the markings but, you can distinguish numbers of fish in a group and no, you won't know what species they are until you fish an area long enough and catch enough fish that your "seeing" to then reliably know what is down there.  Heres a couple of pics from my last laker outing, excellent markings all over the place and I know they were lakers cause I was sitting and jigging right on top of them and catching them.

Some more classic "fish shots" of lakers on the bottom.  The rigger line is at 110', those three markings below are fish!  On many occasions you'll actually see those lines shoot up towards the rigger line, and that could mean a strike!  If these fish continually mark in this manner and your not getting hits, change up hooks and/or speed OR, drop the ball right down in their face!

A big pod of fish, its at times like this that you just might want to try the jigging stuff, it can be productive and a nice change from the troll!  Here again in this pic the rigger line is at 110' whilst those lakers are off the bottom and in the strike zone.  Notice as well that the Zoom on the sounder is set at 2X, sometimes the fish are just lying on the bottom and without the zoom the sounder may not pick them up, and you may be passing over alot of fish!