Hi i’m new on this forum and fairly new on blender too (coming from sketchup…), after some smaller project i tried to make an underwater scene as photorealistic as i could. I know is way far from photorealism but i think i can be pretty satisfied with the results! The two renders are made with cycles (1280*720 250 samples). I also made an animation (30 samples) that you can find on youtube searching “Blacktip reef shark - blender underwater coral reef scene rendering” (i can’t link it 'cause i’m new on the forum…).
Feel free to ask anything, any criticism is appriciated, hope you like it!
Feels “underwater” enough i think, and the shark is well modeled. I do like the clean sand bottom, like you can see in lagoons some times, peoples tend to fill underwater scenes with things usually. What i find strange is the coral reef, looks more like a wall, and the white patches on the water surface, looks like something floating.
Thank you for the reply! Yes, the water is bad, i dont like it too. What do you mean that look more like a wall? Few corals or you don’t like the model?
I mean it looks too uniform in shape. The black shape holding the corals is maybe too vertical, like a wall, and too uniform in color.
Maybe if you change the texture to something more in the tones of the corals, it would look more… well like coral 
i actually don’t like the empty sea bottom at all, it makes the whole scene very un-balanced since you have like a truck load of corals on 1 side, and nothing to balance it out at the bottom. water, could use some improvements, volumn scatter (or absorption, maybe both) perhaps? compositing, maybe add a gloomy fog to it with the blur nodes to give it a more picturesque feeling. nice shark model though
For the scene i was ispired from my last holiday on the red sea last year, where at one point the reef was just pretty much a wall full of corals going down for some meteres in a clean sand bottom with no rocks or coral. My main goal was just to do the shark the whole underwater scene idea came later…