Hello! Geka_ZP, believe, Usco is due to the caustic . I think it would be too typical and stereotyped touch, making emphasis on the fact that the underwater scene.
As for depth, then, as I said, two ways. One material by Fog color , other post-processing , via the render element depth . I describe the difference between a bummer, I just advise postprocessing. If you wonder how to - write. I will tell.
If you do not like it, you can find a generator, for example, the Caustics Generator : the D and make the desired card you own.
You throw it and its image in Photoshop, and then superimposed with the right blend mode layer. For example, Multiply.
If you want a more "correct" result, then you put the card in a Projector Map directional light and shine it on your scene. Only, it is desirable to wish B / W map to the source is not backlit scene😉