I was a lifeguard.
I don't know about the, 'if you're not stressed out, you're not doing it right' point. Sure, it's stressful sometimes, but I'm fully capable of watching one five-year old in 3 foot water without getting my suit in a bunch - and that's all the job is on some days. You learn to love the rain.
As far as an extracurricular, I'd say this: It is as good as any run-of-the-mill job you'd list on AMCAS, but no better. It does not count, in any way, as medical experience. They don't care that you could sit through the Red Cross class on CPR. If you happen to be EMT certified as well, your results may differ. That said, it does show that you're well rounded, capable of looking after others, and have some responsibility, as many jobs would also show.
Worth listing? Definitely. But if you think you can check off 'exposure to healthcare & medicine' on your AMCAS to-do list via lifeguarding, you'll get burned (I almost did 🙂).