Thanks for all the replies, guys. I thought about thermocline, too, but...like Clownfish said, I thought that appears more as a line across the middle, not everything from the thermocline to the bottom.

I only have one graph on the boat. At the console. With GPS, hence the speed. No trolling motor up front. But it *IS* sharing the battery so I'm sure it's interference from that.

Gonna try putting it on its own battery and see if it clears up. I'll let you all know what I find.

Thanks again for the comments.