The unfortunate reality is that coral reefs are declining world-wide. If you could get a side-by-side comparison of just about any reef 50 years ago versus today, you'd see the degradation. I seem to recall one of these photos in National Geographic of John Pennekamp reef in the 60s versus 90s (I think). In the 60s there were these beautiful stands of elkborn coral; that is all but gone now. Our reefs are suffering from a combination of rising sea temperatures, increased nutrient run off, degradation of the supporting seagrass/mangrove fringing habitats, ocean acidification, invasive species, and increased fishing and diving pressure. Individual reefs bear different proportions of these assaults.
One thing I will caution against is comparing fish populations from one year to the next. Fish, like virtually all animals, go through cycles of increased and decreased abundances. I would be skeptical of making assumptions about reef health based on a few years observations of fish populations. These kinds of assessments take many years of systematic record keeping.
With that said, I'm sure Cozumel reefs are not as healthy now as they used to be. Sadly, that's true for virtually all coral reefs.
