The dark area to the left on the top image is a small ledge. It turns black because you are not getting a return from the transducer after the ledge drops off. Imagine you are shining a flashlight in a dark room. Your sonar is only going to show you what the light directly hits. That will help you imagine what it is you are seeing on side image. In the bottom image, it looks like maybe a large log or some other hard structure.
As stated above, it is some kind of structure on the edge of a ledge. Top image also shows some tires in the area. I'd go back and forth a couple times with the down image to see if you can get a different picture.
Either way...it would be a waypoint on my graph...and I'd drop a jig down on it
Your on your boat and floating in air. You have 2 spotlights under the boat. So u look over the edge and see some small bright object but since you are directly over it is hard to judge height. However if you look at the shadow the object casts you get a better idea of its height. SI is just the spotlight.