Create a new layer and change the blending mode to Overlay. Select the round soft brush and increase the size to 700-800px. Set the opacity to 5-10% and paint with black on the margins of the scene and white on the middle. By doing this we will subtly change the focus towards the center of the scene where all the action happens.
Create a Curves Adjustment Layer and drag the curve down to darken in order to darken the scene a little bit.
The surface of the water is a little too bright for my taste. Let’s change that by adding a Curves Adjustment Layer, dragging the curve down and filling the layer mask with black. Now paint with white over the waves to reveal the darkening effect there.
Create a Vibrance Adjustment Layer and lower the vibrance slider to -31.
By the way, all the adjustment layers are located in Layer > New Adjustment Layer > ….
Create a Brightness/Contrast Adjustment Layer and increase the contrast to 39.
Press Ctrl + Shift + Alt + E to create a layer from all the visible layers. Apply a 10px Gaussian Blur (Filter > Blur > Gaussian Blur) and change the blending mode of the layer to Soft light. Add a layer mask and paint the portion above water with black to reveal the effect only underwater.
Press Ctrl + Shift Alt + E again but this time apply a sharpen effect by going to Filter > Sharpen > Unsharp Mask.
Create a Hue/Saturation Adjustment Layer and increase the saturation to 74. Fill the layer mask with black and paint with white over the bloody splash coming out of the beast mouth and a little bit over the boat and the pirate girl to boost their colors a little. You can also paint over the creature head to make it blue and intense.
Open the image “Nebula1”. Select all and copy paste in our scene. Place the layer as shown below.
Change the blending mode of the nebula layer to Color Dodge and lower the Opacity to 70%. In the layer mask hide all the bright areas and hard edges.
Open the image “Nebula2” and do the same operations as we did at step 59 and 60 but this time in the layer mask reveal only the portion above water.
Finally let’s add a vignette to our scene. Create a new layer and fill it with white. Go to Filter > Lens Correction and use the settings shown below to create a vignette. Change the blending mode of the layer to Multiply and if the effect is too strong add a layer mask and hide the places where dark clipping occurs.