There was NO nitrox available for the OP on the boat!(Mod edit)Not necessarily. The drift diving we do here in South FL air divers must ascend early themselves. The Nitrox divers stay down longer. Not sure if that is how it’s done in Cozumel or not. Regardless your computer needs to be set to your proper mix. I’m sure the OP remembered to do that ... but if not it could have led to the issue.
Both he and his guide were on AIR.
What issue are you talking about? Unless you are suggesting that the dive guide's computer was set on nitrox!(Mod edit)