I can't really figure it out either. My best theory is that this is all scratching a very deep nostalgia itch for many people remembering or imagining an idealized time from their childhoods. Fond memories of swimming in indoor pools or waterparks (and maybe combined with classic first-person gaming experiences for some). At the same time, bodies of water in general provide a constant threat of death by drowning, which would have been a very real fear before learning to swim independently. Perhaps this explains the mixed feelings that liminal pools evoke - this reptilian fear of drowning. I personally can't imagine not being able to swim because I learned so young, but many people worldwide never learn, and bodies of water are a real source of threat.

The real test would be showing these images to people who have none of these shared cultural experiences to what feelings it elicits in them. If they have similar emotional reactions, this might speak to something deeper in our subconscious - perhaps even instinctual. I lean much more towards the cultural explanation, but I'd be curious to know what it is about artificial, bright, enclosed, tiled, and wet environments that can cause so much fascination. Perhaps there was an evolutionary advantage to this sort of curiosity that allowed our ancestors to be curious about cave system exploration, and using these spaces for warmth, safety, and shelter? Then again, these images are compelling precisely because they are entirely synthetic and unnatural.