Never went to Cancun or Cozumel but did go on dive trips to Grand Caymen and Bonair for a week each (6 days diving) and never had a stomach problem. Usually at the dive destinations there Is NO TAKE what's so ever of any sea life out to at least 100' of depth. They want to keep every sea creature possible to bring more divers (tourists) to their locations. I saw some BIG bugs (spiny lobster) at both dive sites.
Ah yes,caymen island,big difference in santitation,dang latino's used human waste for produce feritilizer on the island as well,after 3 weeks hanging out on cozumel went out bill fishing with a captain from the key's in cozumel to charter during winter months,my group had disentria bad,our captain had a good supply of pepto for us we weined on the entire day out he lectured us on what to eat/drink,we leaned the do's and don'ts eating out on that island,old gal pal was the dive master at spanish cove on caymen also licensed boat captain up to 80' vessels ....different way of life,they had drinkable well water on caymen,we could drink mixed drinks too.Isla marieta's was the exception for sanitation wilst in mexico,clean well water etc.