I think this is a really nice piece. I like the scaly look and feel to her lower half and the bubbles in the water and it's sense of depth (in more than one way). Also, the transparency of her fins is a nice touch. That's not something I woulda thought to do if I was painting a mermaid. There's just two things in my mind that may need taking a second look at. First is that her right arm seems shorter than the left. But maybe it's just me. Second, there seem to be a lot of light sources. The one coming from the top left of the picture being the most prominent of course. And I know underwater light does flicker a lot, etc. This doesn't throw off the harmony of the piece, it's just something I started to see after looking at it for a bit. I think particularly her left side and her hair on that side too might be darker like the water around it.

Just some thoughts. Great work at any rate.