This effect deepens blues in a sky and darkens clouds a bit. It really needs to have blue sky showing through to work well When applied to water, it gives an almost completely night time or wine-dark look to the blues in the water.
- Select the area of sky that you want to improve.
- Add a Levels Adjustment Layer by clicking on the half-black half-white circle near the top of the Layers Palette.
- Double click on the Adjustment Layer Thumbnail which has two cogs on it and is the first box on your new layer.
- Change the middle number in the Input Levels section to .50 and the number on the right to 240. Note that your histogram above these numbers will probably look different from the one in this screen shot.
- Click OK.
- Adjust opacity as needed.