Ok so maybe I'm late to the party or maybe I'm misreading it but
Is Simon talking to a mutated version of the woman from the other sub?
When I first watched the movie I was under the impression that the Eel just mimicked her voice so that Simon would listen. But on a re-watch I've come up with a new idea.
So when Simon first talks to the lady over the speaker, she seems to want to help him (even if helping isn't what we'd call it). While they talk though, Simon let's slip what happened to Filament Station and the voice seems to build resentment for Simon.
Later when Ava comes down (which the voice knew Ava but that could just be god powers) she kills Ava but seems upset that she had to. At some point the voice asks "Why you Simon" and berates him. (Which she probably only killed her because she was coming for Simon and the black box which would discourage others from coming to the ocean)
The voice is also the last we hear on the black box of the other sub sooo
What if she was manipulated to see the light so when she dies, she can be reborn as the eel. That could make her upset that Simon will be "saved" and Ava won't. It would also kind of help explain why the ocean is human blood and its because all of the monsters in the ocean are human. The ocean hive mind sees this as salvation for humanity, a way they can live without struggling like they have been. It could also explain Simon's rapid mutations and how he survived for so long on a useless death sub.
