If you want to select one item of a set of overlapping items, you can ctrl+click and you should get a list with all items that exist at the point where you clicked. you can then pick the one you wanted.

You can turn shadowing off i think.. you would need to do that in the material lab in the texture of the object that shouldnt do any shadow-casting.

I think adding somethin in the distance would be nice. something subtle to give an extra sense of depth...

http://www.freewebs.com/jabukanews/ThunderballCave-underwater.jpg

Like in the photo, you dont have much particularly defining shapes but it still gives some depth.

Another idea, since post-processing is allowed, you could render a depth-mask (which is pretty much like rendering the z-buffer to a greyscale image) and use it to apply some depth-based color grading.

And finally you could add hints of tiny fish swimming around here and there in the background, even if just two or three of them. It'd help bring a sense of scale (if the scene is scaled properly in general) and strategic placement in distance to the camera could also help enhance the feeling of depth.

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