Our family used to watch Dr Who every Saturday and the episode about the Green Death (the one which looked like slime and would crawl over you and make you glow in the dark and then kill you) affected me for many years.
It took me years (not kidding) before I could swim over a grate on the bottom of the swimming pool without having a panic attack bc that was where the green death came from.
Ironically, I became a collegiate swimmer so I guess I got over my grate phobia at some point.
A childhood belief about swimming pools

others believed things like this
- i used to think that eating carrots would make your eyes glow in the dark— nik
- I used to believe that "Earwigs lived in your ears", scared
me to death for years.— Will Chapman - my best friend was so pale i thought she could glow in the dark— tattiebird
- I used to believe green houses were made of icicles, because that is what they looked like from the road. Anon
- I believed you had to color all the dinosaurs in coloring books green because all the dinosaurs WERE green.— Betty
- I used to think that a "pool shark" was an actual shark that lived in a swimming poolAnon