As a quick followup to my previous reply, this is what I mean:
1. Export your sidescan mosaic to the MBTiles format, with the "add to workspace" option checked.
2. Create a new map project (or use an existing one), and add the image to the project. You can do this by dragging the image from the asset list into the Map View of a project.
3. Create a polygon that you want to use to clip your sidescan mosaic. In this case, I simply created a (very) rough polygon in Google Earth. You could also use a shapefile, if you have one. Import the polygon into a map project using the import layer button:
4. Now open up the layer properties for the mosaic image, check the "Clip image to polygon map layer" option and select your polygon layer from the drop-down list.
5. Turn off visibility for the clipping layer (just uncheck the layer in the layer list).
The image will be clipped:
You could then, if you want, just re-export as an image from the project view, or combine with isobaths, contours etc.