In regards to the water...deeper parts are darker, but I'm sure you knew that. So what you can do is just draw some lines going from lighter around he coast to darker farther out then hit it with a big blur to blend it together. Then you can add a pattern overlay of waves or something like that.
As far as the city goes, it looks like you have the basics down so all you need to do now is vary things up so that it doesn't look too straight. Also vary the colors a bit.