Well, in all my experiences and knowledge, I've never come across any real 'swimming pool full of semen stories'. It's not something that would be acceptable in a civilized society. Pools are supposed to be clean and safe places for people to swim. Such a situation, if it were to occur, would cause major health risks and public outcry. But most likely, it's just a wild and untrue story.
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.