I think you need to use some tilt correction. (This can be done in Photoshop or Lightroom using the Crop tool. Then, hold down Ctrl and drag the mouse over the horizon). You might need to also apply some Lens Correction (Distortion) to make sure the horizon is actually a straight line, and not curved.
At the moment, it looks like it's been turned counter-clockwise a little bit.
For some reason, straightening horizons increases the impact significantly.