I use Humminbird Helix MEGA (1,2 MHz). Generally works perfect with Reefmaster.

My problem is that sidescan imported to Reefmaster has darker and lighter sections (see attached).
I do not see this problem on sonar unit screen.

I can't find any dependence with Humminbird settings, boat speed, engine speed, transducer angle, depth, etc.

I suspect this might be an issue with the automatic gain correction within ReefMaster, and varying depth ranges across those areas. Could you try reducing the AG to zero and see if noticeable differences between the areas remain? It would be quite interesting to take a look at this file - any chance you can zip and share to This email address is being protected from spambots. You need JavaScript enabled to view it. ? (use a file sharing service if very large).