She doesn't feel like a mermaid yet. I really want to see a tail. I'm sure a tail isn't required, but it sure helps. So far, because there's nothing at the bottom, I could see her having two tails, or a shark fin the goes left to right instead of a fin that goes up and down like mammals do. Unless you want it to be ambiguous and just have her be some sort of sea creature, I would answer that question with a tail.
I like the coral red on the fins, could you put more of that color in other areas of the piece? Perhaps in the background?