I like the idea behind this piece and you have a very nice drawing style - I particularly like the girl's face and hair - it's simple and yet detailed at the same time, which is a feature I love in any art.

I like the fact that the selkie blends in with the rock, it makes her seem part of the surroundings in a way that the girl isn't-quite, which is natural. I'm not sure I would have known it was a selkie if you hadn't told me though, as the outline isn't too clear - but if it's an illustration for a book that won't be a problem as you'll have the surrounding story and text to explain it.

I like the different shades you have in the sea. I also like the little touches like the wave splashing up against the rock - you've obviously thought about this. I think that if the wave was splashing against the rock like that there would be some more spray though - you could have added some more flecks of white to make it look more natural.

I also like the subtle way you've shaded, especially on the rock and the selkie. The cross-hatching is nice - it shades but also manages to add texture at the same time. Perhaps you could have done something similar on the cliffs? Just a thought.