Diver-T, you may already have planned your trip by now, but even if you have, Mexico is worth multiple trips especially as a convenient destination from the USA. Since 1999, I've dived in Mexico at least 1 week every year. Unfortunately, I always go in the low season (Spring or Fall), so there's less chance of seeing the giant manta rays or whales which are more common in the Winter. I can't recommend any live-aboards because my wife doesn't dive and is also prone to motion sickness.
Baja California has Los Cabos (Cabo San Lucas, San Jose Del Cabo, Cabo Pulmo). Cabo San Lucas is a bit like Cancun in that it's more commercial, but there are many dive sites close to the Marina. There are strong currents at the tip of Baja (Land's End) where you'll almost always find seals (Seal Rock), but the currents can be quite strong there. Further up on the Sea of Cortez side of Baja, there are some easier and pleasant dives at Pelican's Rock, Neptune's Finger, or off Lovers Beach. There are also wall dives available where you can sometimes see sand falls. Lots of fish, huge schools (entire walls of fish), eel, lobster, crabs, rays, etc. Occasionally, you'll see a white tip reef shark or octopus. There's also a beach dive in Chileno Bay between Cabo San Lucas and San Jose del Cabo. While San Lucas is more Americanized, San Jose has more Mexican ambience. Cabo Pulmo is more remote and secluded with excellent drift dives. The current does all the work, but if you want to stay an look at something longer, you'll have to hang on to a rock or dig into the sandy bottom for a grip.
The Cancun and Isla Mujeres area offers access to many more sites. Large spotted eagle rays are common along with sea turtles and of course a huge variety of other sea life. There are some good wreck dives as well including a gunboat at about 80ft deep. When I was there in Dec 2002, there was a giant Jewfish (a goliath variety of grouper) which lived in one of the ship's chambers. Over the years it had eaten so many unsuspecting fish in the ship that it has grown too large to get out of the chamber, but there's always enough fish going in there to keep alive. Isla Mujeres also offers a national marine park which is great for snorkeling. There are a lot of sand dollars here. The island also has a famous house which is shaped like a giant conch shell.
Cozumel offers world class diving including cave diving. Unfortunately, I only spent 1 day there and only explored a couple of reefs (Yucab & Paraiso). On a future trip, I plan to return to Cozumel as well as explore some of the cenotes around the Playa del Carmen area which is the closest ferry point going to Cozumel.
Puerto Vallarta has a number of interesting dive sites. Los Arcos is a goot night dive destination. Look for octopus and sleeping parrot fish. Sometimes, you can see the protective bubble that the parrot fish exude around them while sleeping. My favorite sites are El Morro and Las Marietas which have small caves, tunnels, and abundant sea life. Altho near the surface, the water can be over 80 F, the water at depth can be relatively cold in Puerto Vallarta even tho it is so far south. The cold thermoclines can be 67-70 F, so many people still wear 5 mm suits. This was at the end of May. However, because of the colder waters, I had the priviledge of swimming with 2 giant manta rays (not the smaller bat rays or mobilas) at Los Anegados. They were simply looping thru the water scooping up the plankton. I was able to touch one and measured them by using my own arm spans. The larger one had about a 14 ft wing span while the smaller one was 12-13 ft. They had the largest remora that I've ever seen attached to their bellys.
Zihuatanejo and Ixtapa offer decent diving, but not nearly as good as all the other areas above. Zihua is famous as the sleepy fishing village in the movie Shawshank Redemption, but it has developed quite a bit since I first went there in 2001. There are a number of nice resorts on both the west and east sides of Zihua Bay The most unusual experience here would be the large seahorses (4-5 inches long) which can be found in Zihua Bay at a site called Caleta de Chon. Caleta de Chon is also an excellent spot for night dives. There are many other dive sites off Zihua and Ixtapa where you can see the usual including spotted eagle rays, but there are no sharks here. My favorite sites are La Piedra Solitaria and Morros de Potosi. Morros de Potosi is the most interesting site with shallow caves, tunnels and lots of sea life, but because of its further distance, lower visibility, and occasional currents, dive operators don't have regular trips there.
