The dangers range from simple environmental, such as wave action near rocks, currents that can suck out towards sea, and in the case of Cozumel's infamous currents...downwellings that can suck you down when diving. Other problems are, yes, equipment failure, ruptured ear drums, embolism, the bends, drowning, etc. These are things proper training will teach how to recognize, avoid, and deal with.
Considering how soon you are leaving, I would suggest sticking to snorkeling this time around. There's still plenty of good stuff to see. I still go snorkeling in such places, and enjoy it
