Hi Yama,
Transparency is not supported underneath raster exports, so everything you see there is the SR image that was exported from ReefMaster. We use the Lowrance land colour as the background colour, which is why you see that colour in all places where you don't have map data.
There isn't really a simple solution to your problem, although with a bit it is possible to change the SR background colour to another single colour. To do this, you would need to export the shaded relief directly from the contour view of your map project and edit the resulting PNG file to add a fill colour to all of the background areas (these are transparent initially, when exporting via the contour view). You would then need to run the raster stage of the IMC map process manually, using the shaded relief PNG file and associated KML file, and combine the results of this with your existing vector export. Quite a lot of work, and perhaps if you need the background lake colour you would be better off sticking with vector only exports from RM?
Matt