Here's a Structure sonar read (these are all from today 14 Apr 07)
Shot of striped bass under bait fish.. Now we were actually able to know the bait fish were large shad as we could see some a foot or two under the surface
Here's a pic of runnin' up the lake, if you look on the plotter, you'll see the boat position just past that elongated jelly bean lookin thing (which is 16 ft rise section surrounded by 26-30 ft of water, kinda like a small underwater hump or island).
Anyway.. you can see how that correlates on the plotter as it shows a "hump", those blue streaks (vertical) are fish suspended on/around that hump. When at running speed, you can see they dont show up as "arches"
yeah, they'll show up good when moving slow or stop'd (note the 2.5 mph), on my boat that's just in gear with the wind blowin kind of speed.
Big difference though when you get up to speed (note the bottom pic at 44 mph), pretty hard to pick out anything other than bottom.
here's a pic for you... show's the relationship of the map to the bottom. here you can see the boat on the plotter is on the edge of a raised bottom, and this is verified by the sonar as you see it going from deep to shallow (rising)
Yeah, when your not moving fast, the returns "can" actually come back like that.
Funny thing is, notice how the arches look alot better on the 50 khz frequency (right side of screen, 200 Khz on left side)? Notice the 2.5 mph speed. This elongated the sonar return a bit, and if a slower or stopped speed, it would've produced that "text book" arch like on the demo screen! A bit faster would elongate it to a point, a running or planing speed would show up as a blip or verticle line.
Though my sonar unit is different from the next guys'.. the idea/principle is the same.
Best thing to do is go by your owner manual and read and understand the unit you will be using, especially as your primary sonar.
Other variables include sea state, when pitching and rolling at idle or drift.. you'll get a "wavy" bottom "up and down, up and down...", not so animated when running and on plane, but can fool folks into thinking they're on good bottom when it's simply wave action pitching and rolling the boat.
50 khz is ok for 50 ft of water or greater,, it throws out a "wider" angle off the transducer.. but it doesnt have the "detail" the 200 khz has.. You can find fish or structure in a wider bottom pattern, but like you said, if you dont fish deep water (what I consider over 50ft.) then a 200 Khz transducer is all you'd need for sure. (dont have to buy in to all the "tackle/electronics monkey" advertised gizmo's)