she just looks like she is chopped and pasted. Try blending her in with the background so she too looks like she is under the water.

Result is pretty ok, imo. Perhaps some suggestions: add more shading to the girl, mostly on her body around the mouth of the shark. This way it will look less flat. Same for the back of her hair, experiment with light and shade. You might want to blur here and there her hair too, gives the idea it has more underwater movement and it's a bit further away (to create some depth of field). Lastly you could try to add a color adjustment layer to make her a bit more blue, that's up to you. Good luck!

LOL