Back in 2005 or so, I used to be a member of a forum where one person would post an image they created, and then everybody else would take that image, do random stuff with it in photoshop in an attempt to make it look better and then we'd all talk about how great we were as artists because we could copy-paste stolen textures into a collage and make it look worse than the original.
Didn't work very well back then, doesn't work very well now, but in memoriam of times long lost, here's my version.
On a more... helpful note... yeah, it's too dark, and I think the water needs some reflective light, or else it just looks like a sinkhole (although I'm not sure how possible that is to do in Blender). And the moon (or earth in the sky?) needs to be... uhm... I guess "shinier" is the word.