I don't think it's air bubbles. I mean, why would they be coming off the bottom at an angle
Wouldn't there have to be some decent current to make the bubbles angle at ~ 45deg ?? And why in the world would bubbles split into a forked dual line ??

What intrigues me more than the angled lines, is that huge honkin' thumbnail return
That's one SERIOUSLY LARGE fish (or turtle) to put out such a large thumbnail return !!! And why didn't the unit put a large fish ID icon on that thumbnail ??

Have you tried dropping your sensitivity down a couple of notches ??
Turning off the Fish ID ??

... cp