I feel like the pose is what holds this piece back for me. It feel's like there's too much gravity involved. As if it's a top down photo done on dry land, not someone suspended in water. Perhaps if you had her drifting across the bottom of the frame, whilst still keeping the head in the same sort of position. Or even swimming down from the surface to tie in with the light source, and the way it draws the piece back. Which moves me onto something else, it feel's like there are two planes in your art. There is the close one, with the 'kappa/mermaid' and has a very short depth of field. Then that suddenly changes to sunlight filtering down through the surface, which has a very deep feeling. The problem is, the way the seaweed is moving and the posing of the fish-girl prevents the two from blending. I don't know how well I explained that, I hope I at least managed to convey some of what I meant. I love the colours
Hmm. To reword it, the light feels out of place. Her pose gives the effect of the seaweed rising up from below her, so I looks like you would see deeper water, not the surface. Another option would be to put more perspective into the light. It looks very round, as if you're looking almost directly up at the sun. Scattering the light in a short line, the way it hits the waves at sunset, would make the surface of the water look more angled, or just elongating the light source further. I think that would fix my only problem with this piece of artwork.
To me it feels like you are adding too much white and desaturating the colours a lot. It's pretty hard to paint an underwater scene effectively but there are few things like floaty hair and bubbles that can help convey the message. I tried a paintover to explain some of the things, hope it helps.
The furthest eyebrow should be more on the edge of her brow. If you imagine her turning towards the viewer face-on, the eyebrow would be half-size and too close to the centre. It doesn't read like it's following the perspective: the eyebrow on the left stops before the corner of her eye, whereas the one on the right overhangs the corner.
Generally, I suspect the whole right side of her face (our left) is suffering from a bit of feature-creep.
The eye that's closest to us is very near the ear. It's possible that the far-side is just at a steep angle (so the whole face is pointy), but if that were the case I'd expect it to be pinched in a bit more, maybe? Or maybe the cheek area needs to be larger to accommodate the eyes? Or maybe the eyes should just be closer together? Or push the ear further back? I'm not sure; it depends how alien you want it to look. Maybe fixing the eyebrow perspective would make the feature-creep look believable.