Diving Mexico

Mexico diving is very varied due to the fact that part of it is in the Caribbean Sea and the other part in the Pacific Ocean

Climate:
Most of Mexico has sunny weather for a large part of the year. Located between the Gulf of Mexico and Caribbean to the east and the Pacific Ocean and Sea of Cortez (Gulf of California) to the west and south, the country's climate is largely affected by these large bodies of water as well as Sierra Madre mountain ranges that extend along most of each coastline.The varied topography explains the different climates of each region.
Average air temperature during the year: 16-49°c
Average Water surface temperature: 26-29°c. Recommended wetsuit is a skin or a 3mm shorty for the hot months and from 3mm to 5mm full wetsuit for the colder months depending on the natural insulation layer of the diver. 
Visibility: 25m to over 60m depending on the region and time of the year.
Coldest time: December to February 
Warmest time: June to September
Possible to dive most of the year
Best time to dive:
Cozumel, March and August 
Cancún, May to August
Mexico's Pacific coast, August to February.
The Cenotes, November to March 
Worst time to dive: For all areas particularly in April/May and September/October visibility can be affected by natural plankton bloom

The Yucatan is known for its famous spectacular cave formations, the world famous cenotes  and the amazing clarity of  water. 
Playa del Carmen has lots of sponges and many turtles.
On the Pacific ocean side the Baja Peninsula of Mexico is famous for encounters with mantas, whale sharks and hammerheads. Isla Guadeloupe is also famous for it's great white shark cage diving.  Whale-watching is also practically a year-round activity here.

If you have a favorite time of year here, one way to secure lodgings for each visit is with a Mexico Timeshare. There are a number of timeshare rentals which offer dive facilities, PADI instruction, and planned excursions. They can also lead you to other accommodation options.

I just wrote a few highlights but you can find more in the regional pages or in the dive center and liveaboard websites.