Carrara Underwater Scene
Here is a basic Carrara Underwater Scene, works in 8 and 8.5. This file creates an ocean surface and seafloor, plus 2 spot lights for caustics and rays. You will need to see http://www.dualheights.se/caustics/ Caustics generator for the useage of caustics images or use your own. I used the supplied shaders in Carrara to set this up, if they don't work, just browse your file structure to find the needed shaders, I have the default structure so it should work for the majority of users. It's up at ShareCG
I took a little searching to find how to do underwater scenes in Carrara and then tweaking to get a nice look, I thought this would help someone to get their underwater scene started.
You can raise the sea floor up for shallower water. Anything close to the camera is a little darker so just increase the intensity of the close up light, it aims just in front of the camera.
I used a couple tutorials-
Mark Bremmers tutorial for the set up and learning to understand how it works. http://www.markbremmer.com/3Bpages/darkarts.html
I Iiked Carl E Schou tutorial with 2 spot lights, one for the ocean surface and the other for the caustics, with the ocean surface starting well forward of the camera