@Navalland OK, so that is your background layer.
In brief:
On other layers apply an image of a greyscale streetmap to the territory outlines and create some cloud-like effect on the seazone-layer. The territory layer should have a high level of transparency, you need to experiment here. When you are ready merge the territory layer and the seazone layer to one png-file, preserving transparency there. This will be your relief-image. Of course you can apply whatever visual effect you want there.
I found this tutorial for creating “clouds”:
https://forums.getpaint.net/topic/17972-make-real-clouds/
Maybe something similar exists for GIMP, too.
For further details please try to reach out to active map-makers. I can only give you this quite superficial information.
