Making realistic underwater lightning


I’m trying to model an ancient/Ordovician reef, and I’ve already hit a major problem that I am consistently unable to solve. Namely, I am completely unable to make my scene look as if it is actually underwater. A picture is worth a thousand words, so:

I mean… I guess it’s not horrible, but it doesn’t look good by any stretch of the imagination. Here’s the .blend file: Ordovician_Reef.blend (569 KB)

What do you suggest I do in order to make the scene look truly underwater?