Like in real estate, location, location,location,
Cozumel, Cozumel, Cozumel!
100 foot plus viz!, Abundance of sea life, fantastic corals, excellent, excellent diving. IT's not one of the top 10 dive spots in the world for nothing. Did I mention drift diving? Way cool!
Great downtown for partying at night if that is what you want, or laid back lifestyle if that is what you want. $1 beers on the plaza.
Cheap!
Stay at all inclusive Alegro resort if you like that, cheap, cheap diving with multiple tank purchases in advance, with access to the best reefs just minutes off shore.
Swim with dolphins at CHakanuba park.
Rent a car or mopeds and explore the north side of the island, have it all to yourself, or discover some of the secluded beach bars over there.
Just came back from both Grand Cayman and Coz, Coz kicks ass!