One thing I'd point out is that your underwater structure has too much 'structure' and too little 'underwater' in it.
You should add a lot more fish, bubbles, seaweed of all sorts, corals of all sizes, etc. Everything that can be seen only in underwater maps and nowhere else.
1. I'd also change the fog color and its thickness, it's really important to get it right. Try to play around with it until you think it looks the way you need it to.
2. Add more things that stretch upwards, it would add the feeling like you're actually under the surface of water.
3. If it's not a playable map, then try to get the right camera angle as well.