Was forced to do a family thing this past March in Cancun (altho I had no desire to ever go to Mexico). We stayed at the Omni which does have all inclusive. The hotel itself was pretty decent and the food was ok (tasted fine, yet I got sick). Snorkelling sucks no matter where you go in Mexico, especially if you have snorkelled/dove in Hawaii or the Barrier Reef. They do not water down the drinks tho
) The beach at the Omni was a good sized one and had decent sand, but was absolutely packed. Group who went was wife & I, father-in-law, step mother-in-law, brother-in-law & his wife, and 3 children aging from 4 to 14. The kids had a blast exluding the oldest. I honestly thought it sucked. I prefer a bit more privacy so I usually just rent a house in the Caribbean or use my parents' condos on Hilton Head or in Florida. Shoot me a PM if you have other questions.
edit - and i believe the Omni Cancun is 5 star