Hi onetankshort,
You are a veritable question machine, amigo, but let's have at it:
1. (Q) What are some dive possibilities in Cancun? We're looking for some great spots to dive. Are there and good wrecks that we can dive from Cancun? What dive operator on Cancun?
(A) Cancun dive possibilites pale next to those in Coz, as you will see. However, among the more exciting dives are the wrecks of the Ultrafreeze & Tulum, but due to depth & currents these are usually reserved for experienced divers. This means you'll need to get a group of like divers assembled & trips are not run everyday. Check with your dive op as soon as you get there & express your interest. There are some okay sites off Isla Mujeres.
For experienced divers, I'd recommend Solo Buceo (on-site at the Camino Real)
http://www.solobuceo.com/default.htm or Blue Peace
http://bluepeacediving.com/ .
2. (Q) What other certifications should we look into?
(A) IMHO, rescue diver is among the most worthwhile of all certs. It provides you with very useful emergency skills & builds confidence. All other things being equal, I want my dive buddy to be rescue cert.
3. (Q) Is the cenote diving something that we should do?
(A) I think so. Here's this little piece I did for my "Ask RSD" column in the Nov '99 issue of "Rodale's Scuba Diving":
"Cenotes ('say-NO-tays'); Spanish for 'caverns') are collapsed openings in the caps of underground cave systems. These are largely limestone structures and mostly filled with fresh water, although some are fresh water on top and salt water below. At one time above ground, over centuries stalactites, stalagmites, columns, petrified trees and flows formed. When the land sunk below sea level, the systems flooded. The most popular cenote diving area seems to be in the Yucatan Peninsula of Mexico, primarily around Akumal. This area contains the world's largest known, and as yet not fully explored, marine cave system.
You do not need cavern/cave certification or any special equipment to dive these cenotes. Your DM will be so certified and dive fully rigged. You will get a brief orientation, dive the rule of thirds, and nearly always be within sight of an exit, even though sometimes it gets pretty far away and small looking. Viz is extraordinary, although there is almost no marine life. The cenotes you will dive are not easily silted, and dangerous areas are well marked and off limits. Some of the best are Temple of Doom, Room of Tears, Dos Ojos and Car Wash. Water temps are about 76 plus/minus 2 degrees, so take thermal protection. Non-divers can snorkel, free dive and lay back in some cool and inviting areas. One thing you should be aware of is the somewhat strenuous trip to many cenotes. The ride in can be rough, and you typically completely suit up, except for mask and fins, and then hike to a steep and sometimes crude ladder entry.
Dives shops in Playa del Carmen, Akumal and Puerto Aventuras can arrange things. If you are staying on Cozumel, most dive shops there can set up a day trip."
The Cancun shops I have referred you to can set up the whole deal.
3. (Q) What type of wetsuit is needed?
(A) The "Ask RSD" column/June 2000 issue of "Rodale's Scuba Diving" magazine carried the following edited item which may provide an orientation for answering your question:
"1. What type of wetsuit will I need to dive the Caymans in March?
Have you ever been out on the street in the spring or fall and noticed people wearing everything from shorts & T-shirts to full coats with hats & scarves, all apparently in comfort? Well, things are no different underwater. This comfort is highly variable among individuals depending on such factors as percentage of body fat, age & tolerance to thermal discomfort. Other determinants, like how many dives a day over how many days you are planning, are also important. When you ask such a question, you'll get answers ranging all the way from 'just a bathing suit' through '5 mm full.'
The question might better be phrased: 'What water temps can I anticipate in the Caymans in March?' You can receive a pretty accurate answer to this, and then can decide what's best for you based on your personal thermal comfort needs."
That having been said, Cancun/Coz water temps in mid-Sep should be around 83, give or take a degree. If I had to pick among my suits, I'd select the 3/2 wetsuit if I was doing multiple dives over multiple days. Of course, others may select differently.
Have a super time down there.
DocVikingo