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It's also a rule applied by lifeguards to people they don't like very much.

"Oh. Hey. That obnoxious kid who throws rocks at bikers and cars went under the waves. Let's go get him."

"Na, wait a bit. 30 Second Rule."

waiting...

waiting...

"Okay, now let's go get him."

By the time the person is pulled up, the lack of oxygen for those 30 seconds combined with the necessary time til rescue will have probably turned them into a catatonic vegetable who won't bother the lifeguards anymore. The moral of this story? Don't Annoy The Lifeguards.