I like the overall layout - nice little nooks and crannies and islands to explore. I agree with JB about the rivers - those blunt ends are bothersome. I'd suggest using the fade function in your brush palette. You can set the length of the taper, the minimum width, and it'll work no matter if you drew the rivers using a brush or carved them out with an eraser.

One other thing that's bugging me is what looks like artifacts from your mountain creation out in the water. Is that intentional, or do you plan to clean that away?