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Regarding Nitrox certification:
If you ended the dive because you were low on air, Nitrox won't help. Bigger tank would.
Nitrox is great for extending bottom times that aren't limited by air consumption and shortening surface intervals. BUT, as you'll learn in your Nitrox class, you also must limit your depth according to your percentage of oxygen. [if I'm wrong, please correct me!!]
Ocean dives are awesome - sounds like you had a great dive. Diving in "less than ideal" circumstances makes you a better diver in the long run! Congratulations!
See you on the bottom,
Andy