I've been to the Cancun area four times. I've never been sick during or after. I brushed my teeth with the water and ate salads and had drinks with ice cubes. I also did not overeat, or drink too much. Most people I know eat and drink alcohol excessively while in Mexico and then blame their stomach troubles on "the water".
We spent our time on Isla Mujeres, an island a 20 min ferry trip out from Cancun, so I can't recommend activities in Cancun. If you have time, a day trip to the island is great. The beaches are nice, the whole place is less hectic and the salespeople less aggressive there. You can rent a golf cart and tour the whole island in one day, with a stops at the North beach, Garrafon Park, and the sculpture garden.