If I'm in the position of that lifeguard, I A) don't want to deal with any actual or near-drownings because I generally like people, B) am going to tell people

If I'm in the position of that lifeguard, I A) don't want to deal with any actual or near-drownings because I generally like people, B) am going to tell people to comply with the posted rules or I'll get fired. If I'm the pool owner, A) I don't want my pool in the news as "the one where that kid drowned", and if it is, I want 100 witnesses where the lifeguard kept shouting at him to stop doing that or he'd be kicked out. I don't want a reputation of hiring lifeguards that don't. B) I don't want to deal with any wrongful death suits. Sure, test your limits, but if you're going to do so, do it somewhere else. Ideally with supervision, but I won't tell you how to live your life. If you look up videos for 'static apnea', WR breath-holders have one or two safety staff in the pool with them in arms' reach.