Duplicate the “Stars” layer (Ctrl + J) and name it “Debris”. Fill the layer mask with black (click on the layer mask and press Shift + F5). Select the standard round soft brush and paint with white (in the layer mask) on the underwater regions of our scene to reveal the specks of white. Change the blending mode of this layer to Screen.
When taking a picture underwater there are blurred particles that appear in front of the camera. Let’s replicate that effect. Duplicate the “Debris” layer and apply a 30px Gaussian blur to the layer, not the mask.
As we go deeper underwater the light gets dimmer and dimmer. Create a new layer and change the blending mode to Overlay. With the round soft brush at 20% opacity paint with black on the bottom of our scene.
Import the light brush (double click on the .abr file). Create a couple of new layers and set their blending modes to Overlay or Soft Light (a weaker version of Overlay). Choose some light brushes from the Brush Presets window (Window > Brush Presets) and create some light rays underwater. Lower the opacity of the layers to 30-40% and use layer masks to hide the light that is visible above the surface.
Let’s create a sun rising above the horizon. First let’s create the background light emerging from the sun. I used a couple of layers set to different blending modes (Color Dodge, Overlay) with a red and a white brush at 20% Opacity to achieve the effect shown below.
Create a new layer and choose the standard round soft brush. Lower the size to 20 px and the opacity to 40%. Click a couple of times in the area indicated below to create the hot center of the sun.
On a new layer set to Overlay paint with a light yellow over the surface of the water in front of the sun. This represents the reflection the sun casts over water.
Naturally the sun should also affect the upper regions of underwater. Sample a color from the underwater section of our scene (a light blue) and paint with the round soft brush at 10% opacity in the area indicated below.
No scene is complete without a lens flare so let’s make one. Create a new layer and fill it with black (Shift + F5). Go to Filter > Render > Lens Flare and select 105mm Prime.
Change the blending mode of the lens flare layer to Screen to hide the black color and reveal only the lens flare. Add a layer mask and hide the bright portions leaving only the bottom of the flare.
The background is done, now it’s time to add the actors in our scene. Let’ start with the usual inhabitants of seas – fish. Open the file “Fish” and make a selection of a fish you like (using the Polygonal Lasso Tool – T). Copy/paste the fish in our scene (and don’t forget to convert into a smart object) and place a fish underwater. Lower the opacity of the fish layer to 13%. Repeat this step 5 or 6 more times but choose different fish and place them at various positions underwater.
Select all the fish layers (hold the Shift key while clicking on each of them) and press Ctrl + G to group. Change the blending mode of the group to Normal and create a Curves Adjustment Layer inside the group (Layer > New Adjustment Layer > Color) with the settings shown below to darken the fish.