Increasing the difference between the foreground and background would make them different enough to describe greater depth. How about changing a combination of variables, like color and edge quality. I see a hint of purple in the background. Increasing the presence of the purple might push the headland further back across the water. How could you use hard and soft edges to create a more potent sense of distance?
Please experiment with these adjustments. The variables can be thought of as a set of dials that you can adjust, labeled Color, Value, Edge Quality, Composition and Complexity.
Send me your versions, which I will put into a slide show.