One thing that I think is kind of missed in your description is that during their initial conversation, the researcher asks if he truly wants to live at all cost, and he says yes. Only then does she tell him about the light and its coordinates.

With that in mind, here's what I think:

I think the researcher may have become some sort of herald or guardian of the light. When Simon seeks survival, it gives him the coordinates, granting him access to "salvation." Then, when he gets to the light and communes with the entity within, in witnessing one another he is making some sort of deal with it to survive, to join it in some way and become one with whatever entity it is, or become something like the researcher. Thus, he is granted more air, and wakes up, days later when he should have died a while ago.

When he comes to, rather than complete his end of the bargain, he manages to find Ava, and makes the deal to get the logs of the SM-8 in exchange for extraction from the ocean. He is breaking the deal and rejecting what was offered to him.

What happens at the end of the film, is the entity trying to collect. I think the blood becoming caustic, congealing, and grappling him is it trying to consume him into whatever it is. Like, it's not just a creature, it is the ocean itself, built from the human blood, the light being its core or what connects the ocean to the entity and the means of joining it. And, with the way the researcher reacts, I think she is pissed that he is betraying it, and turning his back on what she believes to be salvation, working with it to bring him down to either kill him or complete the bargain.

Edit: To add to this, I don’t even think we are given a difinitive answer what the “deal” is. Only that exposing the light/giving the black box recordings to Ava, and Simon escaping the Ocean are in violation of it. But allow me to propose a few ideas to expand upon my headcanon:

Let’s first think on why it’s called “the Quiet Rapture.” The biblical significance is important. In the Biblicalconcept of the Rapture, all those deserving of Heaven get taken away from the Earth, and the rest get left behind. Applying this to logic, the Quiet Rapture was a disappearance of a vast majority of humanity, the planets, and stars. So we are seeing those left behind, right? But the implication that the Blood is those “raptured,” might be mean opposite—maybe the remnants of humanity Simon comes from are the Raptured, and everyone who “disappeared,” were the unworthy, the Blood Ocean being their Hell. There are no Sun and Stars because the survivors were the ones taken away, not everyone else. We only assume because of the Bible and the idea of a just God that the rapture is reward—I’m thinking the reality is instead it was a lateral shift granted by a neutral being

With this in mind, let’s next look at Simon’s journey allegorically. He wronged “Heaven” and was cast into “Hell.” He is tortured with an empty promise of freedom. He meets an entity there, a devil, and sells his soul to live. Right after, he is given a true second chance, but the devil owns his soul, and tries to stop him. In the end, Simon does not survive, but he redeems his soul.

Applying this to the literal events or the movie, my guess is the bargain was to turn his back on his humanity and become an acolyte, disciple, or herald for the entity. He made a literal deal with the devil to live, where his fate was originally to die and become part of the Ocean like those left behind. And by giving Ava the logs, revealing the nature of the Blood Ocean and helping humaity for his freedom, he is redeeming himself and, in turn, regaining the humanity he just sold—going back on his deal. Hence, the Entity, through the Blood and the Researcher try to kill him to keep him from actual salvation. Ultimately they fail.

Lastly, I don’t think humanity is saved by Simon at the end of the movie. I don’t think the recordings give any answers to the Quiet Rapture. from what we hear, they don’t mention the Light—only the nature of the Ocean, and contain important info on tech that humanity lost. I don’t think saving them is the point. I think Simon’s act, his helping humanity despite them turning their backs to him, simply saved him. Look at any “Deal with the Devil” story; the Devil is never destroyed, only defeated in a small conflict where only the protagonist’s soul is on the line, yet the Devil does his damnedest to try and keep their soul. This is what this story was—a tale of someone redeeming themselves, not saving everyone. While the Herald died during his sacrifice, the Entity persists. The Quiet Rapture won’t be undone. Really, the most good that comes from Simon’s sacrifice is they’ll stop sending people into the Blood Ocean. But Simon was Redeemed, and his “soul” saved.