I did my certification dives in November 2008 in Akumal, which is just south of Playa del Carmen, and saw oodles of fish and coral. The Mayan Riviera is a very long coast, so I guess it depends on where you go.
There is also great shallow water snorkeling in Half Moon bay which is north of Akumal Bay. Lots and lots of fish and huge coral formations there!
There is also lots of cenote dives you can do on the Mayan Riviera.
Xel-Ha is also close by.
We also spent a few days on Cozumel, but only managed a couple of dives do to windy conditions. I actually enjoyed the diving in Akumal better, but we were limited to where we could dive off Cozumel because of the wind. We definitely want to go back to Cozumel.
Before my wife immigrated to Canada, she worked as a dive instructor on Grand Cayman, and we plan on going there one of these winters too.