It isn't a problem having the 'junior open water' or 'junior advanced open water' - but I think there are extra restrictions (on depths/times) compared to 'open water' or 'advanced open water'.
With drinking and diving, think of it like driving a car - except that there is no absolute legal limit. I tend to limit myself to a couple of pints the night before, which is probably overcautious - but I'd rather be on that side of the line!
I would aim to not get in the water at the very least until I was safe to drive a car.
Apart from the obvious issues of getting in the water drunk (bad idea...) or getting in the water hungover and feeling like crap, the main issue is dehydration increasing DCI susceptibility, so a pint of water at the end of the night is probably a very good plan.
You should try upgrade your card, I dive with a club and my instructor did it for me, but it cost, not a lot tho.
Just means if you go away of anything you can prove the depths you can go to and five nitric or whatever
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