This image is for a page in a book I'm currently writing. The two robots fall deep into the ocean, and the computer in the foreground will have some information on it.
In the scene I used several particle systems; 1 for the trail of bubbles on each robot and another for the particles in the water.
Compositing is were the magic happened. Focal blur, color correction, glare, gamma, etc. etc.
I was going for a photo-realistic underwater picture. I'm pretty happy with the results, but of course open to constructive criticism.