Need more work, but still pretty good!
Sorry if I'm a bit too harsh on ya, hope it'll help.
-first thing that strikes is a mirror water surface, put some waves (might not be easy but especially when there is high reflection this is important). The image uses a realistic look, and mirror surface like this with huge black clouds on top doesn't make any sense. If this was intended then forget about my comment.. : )
-The rocky surface on the island (brown rocks) is badly mapped (the uv map) or the resolution of your texture is too low, I can "see" the texture stretch and end
-The water has a red-green-blue kinda video noise texture on it, feels a bit weird.
On a final note, yes, there is good stuff in your image, the matte painting is great, it's a nice idea too. Just a bit more work and it's going to be a 10.. ^_^
Keep up the good work!