Scuba diving in Mexico is extremely diverse with the Pacific Ocean on the west and the Caribbean Sea and Gulf of Mexico on the (north) east. Perhaps the most common demeanor is the visibility, which is excellent everywhere (easily over 25 meters).
The most popular area is the Yutacan peninsula, with its Great Maya Barrier Reef, the 2nd largest reef in the world. It stretches from Cancun to Honduras for over 300 km. Cancun, Playa del Carmen and Cozumel are the most visited locations. Yucatan is world-famous for its Cenotes, which are land-based sinkholes with underground rivers and caverns, a must-do for any diver. Yucatan is also known for being the gateway to the Mayan world, so when not diving you can complete your list of one of the new seven wonders of the world by visiting Chichen Itza.
On the east side is the Sea of Cortez, the area between Baja California and mainland Mexico. It offers a great diversity of species (over 800), including sea lions, dolphins, great white, whale sharks, hammerheads, orcas (killer whales), and various other whales, attracting researchers (and divers) from all over the world. The diving is more difficult here due to depth, sea temperature and accessibility, so it is for more experienced divers.
Socorro (Revillagigedos Archipelago) is located in the Pacific Ocean approximately 250 miles/400km south of the Baja peninsula. This region is famous for its big animal encounters and they are often compared to the Galapagos Islands in this respect. It is typically a liveaboard destination and you find mostly very experienced divers here.
Along the Gulf of Mexico and the South Coast / Pacific Ocean there are several wrecks to explore. Acapulco is the most well-known destination for the south.