OpenCycleLandscape map is about nothing in my region. So it's hard to check tracks.
It would be great to add ability to show custom background in map part of Sonar viewer - as .mbtiles, or even as one of projects in current workspace.

You can already do this. Import an mbtiles image into your workspace, then select it from the VIEW/Background Images menu when you are editing the track.

Edit: I should say, you can do this in ReefMaster but not the standalone sonar viewer, if that is what you are referring to.

Thanks, I miss this :(