I just got back from a trip this past Tuesday from there. I would strongly suggest you go to Puerto Morelos...it is the 2nd largest reef in the world only to the Great Barrier Reef. I haven't had time yet to get my underwater pictures developed yet, but it was well worth it. The snorkel trip was for two-45 minute sessions. The first part was anywhere between 6 to 8 feet deep, then we went to the shallow part where there were HUGE elkhorn and many other stony corals. Saw nurse sharks, stingrays, sea turtles, barracuda's, and many colorful fish.
The reef is in a national marine park and is about 20 miles south of the airport in Cancun and maybe 35 miles or so north of playa del carmen(across the sea from Cozumel). I stayed in a resort called El Dorado Royale just north of Playa Del Carmen.
-Steve