First create a new layer.Then use the Preset Manager to import the Light brushes and with a light of your choice create a light above the fish and a light above the diver. To each light add a layer mask (Layer > Layer Mask > Reveal All) and using a black round soft brush at 100% Opacity hide the unwanted parts. Try to achieve something like the scene shown below. You can change the light layers blend mode to Linear Light or Color Dodge and lower the Opacity to 60-70%.
Create a new layer and import the Bubbles Brushes. With white as foreground color paint some bubbles at various sizes coming from the fish mouth and around the diver. Obviously the bubbles should be smaller above the diver and bigger above the fish.
The element that will define the underwater scene will be debris. Notice various underwater pictures and look at the amount of debris (plankton, dirt, little white spots) floating. We will try to replicate that effect in our scene.
To do this we will first create a new layer and fill it with black. Than we will add some noise (Filter > Noise > Add Noise).
You can see the result of the noise filter below. In the further steps we will further increase the contrast.
Add a 5 px Gaussian Blur (more blur means bigger debris) to the debris layer.
Add a Curves Adjustment to make the white debris pop and darken the rest (drag the left and the right slider towards the middle until you obtain something like a star field).
Change the debris layer blend mode to Color Dodge and voila, you have debris. Now you should add a layer mask (Layer > Layer Mask > Reveal All) and with a black round soft brush hide some of the debris and try to achieve randomness. Remember to show more debris in the lighted areas and hide it more in the dark areas. And of course don’t let the debris floating above water level.
After you’re done masking the first layer of debris you can repeat the steps 34 to 36 to create additional debris layers. To achieve larger debris simply increase the amount of Gaussian Blur you apply at step 35. After you’re done adding the debris layers right click on each debris layer and choose Convert To Smart Object. Then apply a 2-4px Gaussian Blur.
In Smart Filters Layer mask (not the Layer Mask) use a black round soft brush to hide some of the blurring effect. All in all try to achieve randomness.
The scene element are all in their place so it’s time for the post processing techniques. Add a Curves Adjustment layer and drag the curve upwards to lighten the scene.
Set the default colors (D) and using the Radial Gradient Tool drag from the top of the image towards the bottom to apply the Curves effect only to the upper half of the scene. You can see how my layer mask looks below.
You can see that the above adjustment make the scene looks better. But we will not stop here. Let’s proceed to the next step and further refine our scene.
Add a Curves adjustment layer but this time drag the curve downwards to darken the scene.
Fill the layer mask with black (select it and with black as foreground color press Alt + Backspace) and with a white round soft brush tool reveal the darkening effect to the left and right sides of the scene. Take a look at my layer mask below.