One possible non - vulgar interpretation could be that it's a very strange or surreal story about a pool filled with something that sounds like 'cum' but is actually a non - offensive substance. For example, it could be a made - up story about a pool filled with a creamy - looking plant sap or a white - colored non - harmful chemical compound that someone humorously named 'cum' for the sake of an absurd story.
We could change 'cum' to 'foam'. So the story could be about a pool full of foam. Maybe there was a crazy party at the pool and someone accidentally spilled a whole lot of bubble - making solution into the pool, and then all the kids had a great time playing in the pool full of foam.
I'm not sure which specific 'sisters trapped in pool' story you are referring to. There could be many such incidents. It could involve sisters accidentally falling into a pool, perhaps due to a lack of proper supervision around the pool area, or some sort of accident that led to them being unable to get out on their own, like a pool cover malfunction or slippery sides.
Mostly, it is hygienic. The chlorine in the pool is there for a reason. It keeps the water clean and free from many common pathogens. But, if the pool's filtration system isn't working well or if the chlorine levels aren't properly monitored, it could potentially lead to unhygienic conditions.
There may be a vast amount of fanfiction out there, but this particular description involves inappropriate and vulgar content. Most fanfiction platforms have rules against such content. So, it's improbable that there exists a fanfiction with this description. Fanfiction usually aims to explore characters' relationships, adventures, and development in a more appropriate and respectful way.
There are no reliable reports of such things happening in normal swimming pools. Swimming pools have filtration and cleaning systems to maintain hygiene. And such a situation would be a serious violation of public order and good morals, so it's probably just a fictional or very rare and abnormal anecdote.