Okay, so I'm not entirely sure how this will be recieved. This is a fairly dense SCP concept (in that it's not just An X That Does Y), and the last time I did one of these, it didn't go over well.
So yeah, as I typed this up, I realize it's becomming a wall of goddamn text. So I'll hide it. But please, do read it if you have the time. Like I said, this is an SCP that came together as a result of multiple vectors (which contributes to its complexity) in a way that I think works. If it doesn't, well, I won't be surprised.
The long-and-short of this SCP is based on a few bits and pieces I've seen in movies, mixed together in such a way that I don't think is a full-on copy of any of the inspirations.
The initial core of the idea came from the movie Moon. I'll try not to spoil the film, but ultimately a man learns that not only will he not be going home after his job's contract is up, but that he'll be doing said job forever… and he'll never realize that he's doing the job forever. The thought really appealed to me as one pretty damned creepy.
But I didn't want to just copy Moon, so I took a bit of inspiration from The Thing, in which nobody is really sure if they are who they say they are. That guy could literally turn into an alien monster next to you, and you would not know. In this SCP's case, I wanted to twist that around: that you might not be who you genuinely think you are. The entity that took you over sets things up so you're never exposed to that truth.
So, combined with seeing a film about ants / aphids, I came up with this SCP. People are sent down to this underwater creature that either evolved or was engineered to be taken care of. It's 'taken care of' by absorbing and assimilating a small amount of another species, which live and die (only to be regenerated) without knowing that it's really a copy.
TLDR: I thought the idea of a Foundation crew constantly repeating their lives, and generally not knowing it because all of their personal growth is wiped out when they 'repeat' interesting, and tried to turn that into an SCP.
Edit: Before anyone says it, I know we have some somewhat-large underwater SCPs already. I didn't really picture it as one, but it's really the only place I could fit something that big.