Under water scene using Photoshop.

 The base image I used for the project is a close up of some coral with a fish within it.

The first edit I did was to, using the stamp tool, remove the fish from the image. I then used the curves and the levels to create areas of darkness and adjusted the contrast to add brightness to some areas.

I then added an image of a scuba diver to the background and gave it an "overlay" layer style, I also applied a mask to it so I could hide the right side of the image. I also adjusted the levels of the scuba diver and reduced the opacity. Using the bush tool and motion blur I crated some light ray coming from the direction of the diver and reduced their opacity so they are very faint in the background.

I then placed some coral in the foreground and slightly blured it to give it some depth for when I later added in the text. In the left hand corner I duplicated the background to put it in the foreground so later the text can sit behind it.

 I then added the text "LIFE" in a font called "Molot", to this text I added a large outer glow in green and a small drop shadow to give it some depth. I then added a layer mask of an image that I created to give the text some texture. Above the text I added some light steaks using the brush tool and motion blur,  I reduced the opacity then using the blur tool made it look like the text is giving off some kind of chemical.

I then added some bubbles to above the text to emphasise the idea that it is giving of some kind of chemical. I also reduced the bubbles opacity and added a layer mask so that I could hide some bubbles and emphasise others.

 To the entire image I applied levels and curves adjustment layers to darken certain areas.

Next I added some mutated fish and some more coral to the right side of the image. To these I then applied levels adjustment layer masks .

Finally I applied a green overlay to the rock in the foreground to make it look like it was covered in sea weed. 

Image sources:
Font: Molot (http://jovanny.ru/)
All foreground, background and base image: http://sxc.hu/