Some tips for underwater scenes:
Blur, Particles and lighting falloff are the things that give that sense of depth and location when underwater, and vary VASTLY depending on depth and time of day, as well as the quality of water, current and the surface (wavy or just rolling swell or choppy etc). Use these things to locate the scene.
Right now I don't know what time it's set, how deep it is, or where I am (tropical bay, or mediteranian sea, or something).
Colour falloff is linked to the above, the way reds and greens disappear, as well as the 'haze' of scattering near the surface helps give some pop to the scene.
Materials need some work, especially blending. Sand is easily picked up by current and will settle in crevices in rock. It also doesn't usually transition straight and cleanly to something, it can build up around the edges, and is affected when it's in the wake of an object in the current.