Hi Alain,
Sidescan mosaic KML exports are not simple "ground overlays" (a single image with an extent defined in the kml file). They are actually comprised of hundreds (or thousands) of separate image "tiles", which are referenced by a tree of kml files. As such, they are not support for import into ReefMaster.
If you want to view a mosaic image within ReefMaster, choose the MBTiles export option. If you also check the Add to Workspace option, then the image will be automatically added to the current workspace once created. If you want to add the image to another workspace, just import a new background image and select the .mbtiles file, which can be shown in RM with no further processing.
