According to Word of God, the creature at the end is none other than Cthulhu himself. At first glance, one may think otherwise due to the creature neither looking like him, nor having any of his psychic powers (specifically, telepathy). But it makes sense that neither apply in this case for two reasons:
1. One of Cthulhu's powers is that he can alter his form as he wishes. These altered forms still look like Cthulhu in general outline, as the creature does (i.e., an enormous humanoid with facial tentacles and additional limbs on its back).
2. His psychic powers do not work underwater. And even if they did, he didn't use those powers when attacking humans on the surface in Call of Cthulhu, so he may have other reasons for not doing so.
When we first meet Captain Lucien, he says that he managed to send up 22 escape modules from the Kepler Station. But at the end of the film the newspapers articles over the credits indicate that Emily and Smith were the only survivors of the incident, meaning that all the 22 modules launched from Kepler were intercepted by the creatures and destroyed.
Actually, looking closely at the two lines of text below the first headline that aren't blurred, they are referred to as "two of the last remaining", making it possible that some of the others might have have made it but been rescued earlier, and that they weren't mentioned in the subsequent articles due to never seeing the monsters and thus having nothing to testify about them.
By the end, one must wonder what happens to Smith and Haversham. There's no way Tian Industries would let them go or let them live now that they know that there are creatures living under the deep.
In his video about the movie, FoundFlix points out how weird it is that Tian Industries intended to resume deep sea drilling, even after logically have known about the creatures, and he theorizes that this means that they were not only aware, they wanted to find them.
Along those lines, YouTuber Deusdaecon has theorized that Tian Industries is run by cultists and it's possible that the people in Nora's facility were meant as human sacrifices to the Eldritch Abomination citing how less advanced the main facility was while the drill was far more advanced and intact in spite of being older. He also points out that "Tian Industries" was so named because of Lovecraftian, according to Word of God.
At the end of the movie, in the main drilling facility, the sound system has a repeating message saying "TIAN INDUSTRIES, WE HAVE BIG THINGS IN STORE FOR YOU" while the Cthulhu is in plain sight. That's pretty blatant.
With the reveal & the Word of God confirmation that the creature is Cthulhu in the finale, that leaves the door open for other characters and creatures from Cthulhu Mythos to be apart of the universe. As well as the possibility that several of the stories of H. P. Lovecraft were documentaries or recorded accounts in their world, either covered up previously or just simply not believed by the public when released.
Considering that it's revealed that the Cthulhu Mythos is involved, the ending makes sense as it's not the first time a nuclear explosion is used against the creature. In The Trail of Cthulhu by August Derleth, a nuclear bomb was used against Cthulhu. However, that story implies the victory is short-lived. If that's the case for that story, considering Tian Industries plans on getting back to drilling as soon as they can, it's clear that it's not the end of Cthulhu as viewers may think.
To avert the Lord British Postulate, most Cthulhu-themed tabletop role-playing games, while providing stats for a corporeal form for Great Old Ones, note that killing them (itself next to impossible given how high those stats are) does not actually kill them. . . you might discorporate or destroy their current physical body, but their essence lives on and they will reform. Indeed, in The Call of Cthulhu, Cthulhu is temporarily stopped by being rammed with a sailing ship, though the damage done is implied to be more of a minor inconvenience. Meaning that yes, even a point-blank reactor meltdown won't keep him down for long.
Even if more people had managed to survive the explosion and the trip, they had no way of getting more people to safety than they did. In the end, there were only two functioning escape pods.
If the monster at the end was Cthulhu, and the creatures attacking were his Deep Ones, there's a very unpleasant conclusion lurking. Deep Ones do not breed true with each other, hence their penchant for establishing surface cults and luring humans into worshiping the Great Old Ones. The Deep Ones breed with the human cultists, siring children who appear human but slowly transform into Deep Ones as they age. Not only are all the attacking creatures potential former human beings, if the Deep Ones capture any of the rig workers, their fate is likely to be very unpleasant. Perhaps it's a good thing the humans are so deep in the ocean the environment is instantly fatal.
