In this step we will create the caustic reflection of water on the bottom of the sea. First create a new layer (Ctrl + Shift + Alt + N) and choose Filter > Render > Clouds. Make sure you have default colors set (press D to set default colors).
With the same layer selected choose Filter > Render > Difference Clouds. This step will give us this interesting effect shown below.
Press Ctrl + I to invert the colors because we want our underwater reflection to be white.
Let’s increase the contrast a bit and make the caustic water reflection more pronounced. Add a Curves adjustment by choosing Image > Adjustments > Curves and drag the left slider towards the center.
As you can see in the image below the effect looks much cleaner. This image could also be used to create Lightning effects. If you want to create Lighting effects in your other projects simply change the layer blend mode to Screen and the black parts will disappear leaving you with a nice Lightning effect. After that simply add a layer mask and hide the unwanted parts. But let’s continue with our underwater scene and proceed to the next step.
Change the layer blend mode to Soft Light and press Ctr + T to enter Free Transform. Resize and position this layer like shown below and press Enter after your finished. Starts to look better and better, ehh?
Add a layer mask (Layer > Layer Mask > Reveal All) and with a black round soft brush at 80% Opacity gently mask away the upper edges to make the reflection fade away in the distance and blend it seamlessly with the scene. Remember, in scenes where you want to achieve realism hard edges are your enemy, especially if you are doing nature scenes.
Open file “Fish” and using your favorite selection tool make a selection of the fish. Copy and paste this selection into our document. Position the fish like show below and using layer mask or the Eraser Tool at 50% Opacity partially erase the tail and the upper membrane. Fish membranes are usually semitransparent so this step will bake the fish blend with the scene better.
Duplicate the “Fish” layer and Ctrl-click on the layer to make a selection. Fill this selection with black and press Ctrl + D to deselect. Apply a Motion blur to this layer to mimic the shadow below the fish.
Change the blend mode of this layer to Multiply and lower the Opacity to 56%. Add a layer mask and erase all the unnecessary parts (the areas above the fish) by painting with a round soft black brush tool at 100% Opacity.
In our scene the light is coming from the upper left side so let’s add some lights and shadows to our fish to make him integrate into the scene. We will add some darkness first. Add a Curves Adjustment Layer and modify the curve like shown below.
Add another Curves adjustment layer and modify the curve like in the image below. This will further darken the scene.
Now for the lightness part add a Curves Adjustment layer but this time drag the curve upwards to lighten the scene.
Now that we have the lightness and darkness layers created simply fill all their layer masks with black. Next select a round soft white brush at 80-100% Opacity and start painting in the layer mask to reveal the desired effects. Paint with white in the layer mask of Lighten layer to add light to the scene and paint with black in the layer mask of Darken1 or Darken 2 to add darkness to the scene. This step it is more about experimenting and trusting your inner artistic eye than about precise adjustments. You can see the results I achieved below but don’t worry if you don’t have the same result. Just keep on experimenting until you achieve the effect you desire.