Beautiful image! What medium is this--colored pencils? If so, I'll just go die of envy now... j/k, they're not my forte, but I am so impressed when I see the work people can create with 'em. The lighting on this image is great, the colors are so vibrant. The only thing amiss is it doesn't seem that 'underwatery', aside from her pose and fins. I'm no expert on making underwater effects, though, I apologise becuase I cannot give you any advice on that.

Anyways, as amazing as the coloring and shading on this piece is, my favorite part is her face. The expression seems almost like she's curiously surprised, as if she thinks the skull is a new kind of shell for her to collect.

Overall, this is an awesome iamge--I have to commend you for working so hard on a detailed background. So many people skip over them, and it really does make an image great!