I had mixed feelings about this one as it seems well written, yet pretty familiar. It's remarkably similar to an earlier Pseudopod story by Eugie Foster, Biba Jibun - the one set in Japan where the girl eventually realizes that her mother really was a spirit/demon-thing, and the theme is not so different either as it dealt with the treatment of women in a society as well.
This story has a similar setup and execution - another teen girl discovers her family members have abusive ulterior motives in their actions and a shady past, and that the family legend of her being born of a non-human mother actually is the truth after all. A deadly revenge is enacted on one or more of the icky male characters, and the girl gets to meet mom and discover the whole Truth.
The big divergence in the two stories, aside from the mother only getting a sort of stand-in here, is that, in the Biba Jibun story, the character gets to run off with mom into a sort of happy ending, while here, though the author notes that she is well of aware of that kind of ending for this kind of story, there will not necessarily be such a simple happy ending here. We still get the deadly revenge fantasy though, and the mother stand-in is another grinning and knowing creature who reveals truths while offering alternatives to how the world of the character is through magic.
I'm not sure which ending I prefer. I too like the idea of the wound that won't heal, as it seems to hint at both lost innocence and impossible to reconcile realities. Even with the similar just deserts element, this ending feels more realistic in that way, recognizing that there is no complete healing from such realizations as what the character has gone through. But then again, I liked the main character's grab for empowerment and the even more mischievous mom-thing in Foster's story more.
In both stories, the girls seem to choose to side with their inhuman ancestry, and let the humans who bug them die, as a sort of rebellious act. In comments on the Biba Jibun story, people questioned that a little and wondered if the character had really found wisdom or empowerment, or had just given up and gone with being a demon. Here, the thing I'm finding troubling is that the other people on the boat, some of them teens like herself, are not much more guilty than the main character is, as she has been in the same trade for a while too, and they don't seem to fully grasp the implications of their system any more than she did until being just recently enlightened about her ancestry, even if it's arguably a willful ignorance. Yet we are seemingly supposed to just accept her absolute judgment placed upon them as an absolute justice. It doesn't quite follow for me, as she did all the same stuff to the "fish", short of molesting them, as her colleagues for quite a while. But she is part "fish", so she gets a pass as she can pick sides? For a story that is supposedly about learning to not pick sides against the "other" (we are told in the outro), that seems a bit convenient. But then, maybe it's not supposed to be taken at face value.
Lastly, it's kind of an interesting inversion of Innsmouth, in that instead of the "fish" coming from the sea to mate with the humans, the humans go to the sea to mate with the "fish."