Very hard return--concrete or metal. I'd guess a piece of bridge or sunken car.
Dark orange/reddish hardest return
Then yellow
Then cream colored
Then green colored
Then blue softest return
So, your 2D image shows a surface of an object with hard return signal in the middle (yellow) with a harder return (dark orange around the yellow) to a straight edge on two sides, with a soft blue return (algae/silt/vegetation/bait fish) sloping off the sides.
To get a better picture, you would need 455/800 kHz DI
I agree, that's a big block of concrete or something of similar density. I often see the speckled black/white 45 degree image right next to a steep vertical drop of a solid surface. I don't recall ever seeing that for what I knew for a fact was a brush pile. The sloped side to the right may be a software-interpretation artifact, something that's not really there.