It is highly unlikely you will ever see an arch is 5' of water - the sonar cone is only roughly 1/3 the water depth (200kHz beam), so you are looking at a spot only 2' wide on the bottom, directly under your boat.
A suspended fish has even less cone than that, and most fish will move out of the way if your boat passes that near in such shallow water.
Regarding the bottom hardness - yes, the wider the red band, the wider the area that your sonar is returning a strong signal, which is indicative of harder bottom. But you have an SI unit, which will tell you more about the bottom content than any 2D sonar could.
You will see more arches with the 50kHz beam, as I recall the beam width is roughly equal to the water depth.