Some seriously different responses to the original post, I think it truly depends on what your doing / intending to do realistically, in some places you can’t get taught anyway, however if you have access to decent education then really one should plumb for it, that said,,,,
Who taught / certified Cousteau, Falco, the Haas’s, buster crabbe etc,,,
And I have to be honest here I have a friend that has done 100m dives on a homemade rebreather who’s sole certification is Open water (padi thingy) I’m going to expand on this particular character, he’s an engineer that understands gas theory, physics, blood flow etc far better than me, and I’m one of the few that knows of his lack of certification if you saw him in the water you wouldn’t know, (better than many instructors) of course in the uk he isnt really in any dire straights as a chamber visit wouldn’t result in a bill or proof of cert, and as he mainly solo,s he isn’t really liable to anyone else,
I’m not necessarily condoning his habits or rationale but just wanted to demonstrate that someone with full understanding isn’t necessarily going to die immediately upon entering the water..