First off, this image reminded me a lot of the Deep Sea World here at Queensferry in Scotland. It has an underwater tunnel just like this that you can walk around to see all the aquatic life, and also watch the sharks being fed at certain times.
What makes the image for me, is definitely the capture of the sting ray, along with the shadow of the human underneath. It really gives the photograph a great sense of scale, and character with the two combined. The composition works well in my opinion, there is a lot of dark, empty space, but I don't think the dead space necessarily takes away from the overall image itself. If you decided to crop out come of the left, then it would mean the tunnel light, with the human shadow was too centered, which I wouldn't like as much as it is at the moment.
Technically, the shot is sound, the exposure is accurate, the tones suit the image in my opinion, and gives it all that more depth, and contrast.
No real suggestions from me to improve this shot, apart from perhaps finding a different part of the tunnel where the large box (entrance/exit for divers I'm assuming) is not present, as I feel this is the only distraction in the image.