I was also a kid doing this, my cousins and I held ourselves underwater with the ladder rungs in a swimming pool.
At first, yeah a minute was tough. But then it rapidly increased. Unfortunately I don't remember where we topped out, but I think ~3 minutes.
We would also swim pool lengths underwater(but it was a relatively small pool at a condo building). I think I swam 9 once.
They'd let us stay out all night at that pool, it was great. Florida summers don't really get chilly.
But I didn't at 14 years old, nor did my mom or uncle, apparently.
We did spot for each other, even then, but I don't ever remember anyone thinking they almost passed out or anything.
But it was on the other side of the pond!
Pretty demoralizing to be labeled a cheater after such hard work expanding lung capacity and efficiency. After that, I wouldn’t even try anymore just so I wouldn’t be called a cheater again. Quit band the next year.