Most of what you asked has been covered pretty extensively in other threads but here are the highlights
1). The island and state governments do take Covid very seriously. They had a 2 month hard lockdown (including a 5 pm curfew at one point ) that ended in June and then gradually did a partial re-opening. Curfew is 10 pm now , restaurants are still operating at reduced capacity. They have extensive mask wearing, social distancing and sanitation rules. They check your temperature, give you a squirt of hand sanitizer and require a mask to go into the grocery store for instance. Compliance at most tourist oriented businesses is pretty high, but there is still Covid on the island and in the state in general.
3) Mexico in general has only been testing the very sick. This has kept the top line case counts somewhat low , but the positivity on tests is sky high meaning it is obvious they are missing most cases. The total death count is actually 3rd in the world behind the US and Brazil and the death count here has been shown to be understated when press organizations have looked at actual death certificates
3). Dive boats are allowed to operate at 50 percent of listed tourist capacity (6 pax boat equals 3 pax now )
4). Planes are rather full. There has been extensive discussion on this. High air turnover rate and HEPA filters keeps virus from building too much in the cabin air , but if you are literally sitting right next to a person with Covid risk has to still be there. Bottom line on planes is risk is way lower than you would think , but not zero.
The general takeaway is that you can visit Cozumel relatively safely if you follow the protocols as required by law If you ignore the local rules and act like virus doesn’t exist there are plenty of chances to catch the virus