And, here's the final result.  I had to really bump up the HDRI background to get the waves to show well. In fact I punched it further in Photoshop. 

I created a download file of this scene but swapped out the assets for legal reasons. So, the render of the water looks better in my opinion in the uploaded one. (reminder that the water texture uses Digital Carver's Guild Shader Ops Invert function - if you don't have it the waves will look different but probably ok)

So, this is a scene where I use the Realistic Sky settings in a completely different way in order to get the deep water effect – that fading into the distance thing. I've got some junk floating in the water. This is done in Photoshop. However, for an animation, I'd totally do this with a particle generator. 

You can get the Carrara file here: Underwater Scene

And if you want to see the stuff I do in a Photoshop file, here you go: Underwater Photoshop