It's nothing to do with boats but PADI (and perhaps some other recreational agency) Cavern cards are not always accepted for cavern diving or to meet the prereqs for some Intro to Cave courses.
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I have been using my cardboard, laminated, University of California Research Diver card (expired on 1 November 1974 to boot) since I received it and have never had a single problem with boat access, tank fills, etc.
Some agencies do not put pictures on cards. My YMCA, SEI, CMAS, and NAUI cards do not have pictures. ....
I carry my trimix card. Had a guy in Bonaire question whether or not it was good for a nitrox fill, so I told him to just give me trimix without any helium in it. He thought about it for about a second and busted up laughing ...
... Bob (Grateful Diver)
Naui Scuba Diver is equal to Padi OW Diver. PADI Scuba diver is equal to Naui passport diver. So yes, Mr Johnson will be able to dive anywhere, and I'm sure he did in over 30 years of being a diver.
I'm not an expert by any means, but I would tend to think that the main advantage to having the same cert from different agencies would be gaining exposure to the different teaching methods from different agencies. I believe, but can't swear to it, that when I finished my OW cert in MX, the folks I was diving with would let you get two cert cards at the same time if you passed the tests for both agencies, I.E. SSI and a PADI cert. I didn't do it, but in retrospect, if it really was an option and not something I made up in my own little head, I probably would have, just for the heck of it.
Kristopher