I'm gonna throw out a guess and say you have your sensitivity turned up too high
Without knowing how deep the water really is, you may be getting echos if it really is only 8.5ft deep. But, if it's deeper than that, whatever is causing the swath of red is so dense that it's blocking enough signal to make the unit "think" it's only that deep.
I've run across a school of Shad that was so thick that my unit read 10ft deep (top of the Shad school) when I was actually in 25fow.