It's looking nice. As regards painting with a mouse, I really think you'd have a much better time with a tablet.
I think I'd use a different color for the coastline stroke--a brown darker than your land, but not too dark. That way, you can put city markers down on the coast and they'll be visible.
Are you planning to texture the land? Right now, the only texture in your painting is the water, so it jars a bit with the rest of the style.