Claymore - If you get the chance, dive in the Costa Maya. I believe you will enjoy it if you book with Dreamtime Dive or if you make arrangements with Selma N. of Nomad Divers. ( see www dot nomaddivers dot com]
Ever since I got certified in 2000 in Huatulco I dreamed of diving Banco Chinchorro (almost directly east of Majahual). In 2008 I took advantage of a special Dreamtime had for an unlimited diving package. Although I did not get the opportunity to dive Chinchorro on that trip (high winds, big waves make the 2+ hr. trip from Xcalak a little "rough") I returned in 2009 (dove Chinchorro!!) and have another trip planned for this July.
The shops that cater to the cruise ships tend to dive sites close to the new lighthouse/crusie ship dock and slightly south of "downtown" Majahual. (see dive sites on Nomad's website).
Of these "northern" sites, my favorite is Escalones (on the ocean side; in 2007 Hurricane Dean covered the shallower land side with sand and other debris from the beaches). The top of Escalones is about 40 ft and the "wall" of coral slants sharply down to a sandy bottom of about 90 or 100 feet (I never felt I needed to go to the bottom). You could spend an hour at 60 feet casually swimming and poking through things and not see the entire site.
Like Cozumel, there are many varieties of hard and soft corals, sea fans and other life that resides on the coral mounds. There are plenty of finger grooves and sand chutes to explore and look for critters. In July we spotted turtles on almost every dive as well as sleeping nurse sharks, morays, large schools of grunts, snapper and porkfish and lobsters.
I prefer diving in Costa Maya to diving in Pto Morelos, Playa del Carmen or Cozumel (I've made multiple trips to Czm, PdC, PM, & Akumal between 2000 and 2007). If you dive with an independent instructor/dive guide like Selma of Nomad Divers you might get the opportunity to do leisurely dives over reefs much further south that are less frequented by the shops closer to downtown. These are what the reefs of Puerto Morelos, Playa del Carmen and other "towns" to the north looked like before they were trampled, dredged, overfished, starved or suffocated.
Hope this helps you make your decision...