S.W.
I've been to Cancun and Isla Mujeres (island off Cancun) four times. I've never gotten sick. I took my 9 year-old daughter there last March and we didn't get sick. We only ate at tourist restaurants, no street vendors or restaurants that the locals frequented. We did brush our teeth with the water. You can buy a fruit wash, to wash any produce with.
My ex, however, got somewhat sick the first three times we went together. We ate at the same places, similar foods, etc. So, I could only assume that some people are more senstive to travel and changes in diet. My mother got diarrhea every time we traveled within the US!
I talked to a Mexican man who said that we come there for vacation, eat much more than normal, different food than normal, and lots of alcohol, then we blame the water!!