PADI

Padi stands for Professional Association of Diving Instructors.
it is the most popular SCUBA diving certification agency. You pass their course and you get a pretty card and are allowed to dive.

Among divers, there's a joke about the real meaning of PADI:

Put Another Dollar In

As a professional association that's what you'd expect...

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(Anyway, PADI is the best diving certification agency for recreational diving, so it's fair to pay for a good service.
And, sigh, I am a PADI member!...)

PADI, while a great association to be certified with, has earned a certain measure of scorn in the diving community, the foremost being the aforementioned "Put Another Dollar In" gag. A great many also feel that PADI is the way to go for amateurs and tourists, but more serious divers might be better off with another certification. Having been certified with both PADI and NAUI, I can safely say that the NAUI open-water course was more stringent and covered more ground (or water, yuk yuk) than the PADI equivalent. I also tend to like NAUI dive tables over PADI, for some reason.