I think you two have very similar posts about this and I think (IMO) that this is one of those alternative stories. For me, I always enjoyed those CYOA books that take this one aspect or part and basically throws in something a bit extra. Granted, they should be able to explain the monster stuff a bit better, but maybe when we actually get to the story, that could be something that comes up.
A fish girl that was an experiment in a science lab could be a world of explanation, especially if your father had something to do with her release or his debt requiring him to let her go. But I digress, that's just me speculating...