You need to practice details with the height brush. Don't just use the larger brush sizes.
Read
~Void~'s tutorial on Natural Environments. Chapter 2 should help you a lot with height variation, but the entire tutorial is worth a read for environment details and tile variation.
Also, when the water takes up as much space it does here, you kind of need to add some details to it. It looks pretty shallow, so even details under the water surface should be visible.
You can add rocks, floating debris, shrubs, etc. Let some of it float atop the surface and lower some just beneath it so it gets tinted by the water colour.
The fog is done well though. It's a solid start, so keep building on this.