I guess the double line marks the closer limit for a null sector. Next to the 'line' there is a narrow dark area that I think is caused by a null sector. I have no practical experience from the mega-systems, but I have seen this kind of phenomenon many times with other SI-units and SI-transducers. The dark bands use to be narrow and extra dark when the null sector points to the seabed almost directly below the transducer and when the seabed leans a lot, like in the direction of the beach.
A null is a direction in the beam pattern in which no sound is transmitted. One or more nulls will appear in the beam pattern in almost any sidescan transducer. The higher the frequency, the more nulls.
If the surface water has something in it that can cause haze and if the upper part of your left beam is blocked by something, that might explain the absence of haze in the left view.
Rickard