This is just a personal preference, but I think beams of light (volumetric) piercing through parts of the water would look cool. In particular, maybe some striking the main focus (coral "tree") of your scene since technically, I don't think you'd see all those brilliant colors without a light source. I'm also thinking that the darkness and haziness of the water should maybe be increased as you go into the distance? I don't know for sure since I've never been scuba diving. It's just the impression I get from watching PBS nature shows :) Can't wait to see how you progress with this though. Looks like a cute idea!