You know what's up friend! Yes, even though Cozumel is pretty much next door, I haven't been there yet. I do travel to Mexico City quite a bit for urban travel, but I will definitely make time to visit some of the spots in the Mayan Riviera in the near future.
Have you every considered Tikal?
That might be too inland for my pale blood though I really want to hit Belize for some diving some day.
Absolutely nothing to worry about. You'll be fine!
SCUBA in Belize is top!