Lynn, you're in the middle of some great diving along the second largest coral reef in the world. Not every spot is equal, but they're all very good. First choice requires travel, check the dive shops in Playa del Carmen for a trip to see the Whale sharks, snorkeling only. . .west of Cancun near the island of Holbox. Some shops leave from Cancun with a longer boat ride, some send you to Holbox island and a shorter boat trip out to the Whale sharks.
Click on the Mexican topic of this thread and search for the good reviews several shops are mentioned frequently, or search for the shop in your hotel.
If you are an early morning person, catch the first ferry from Playa del Carmen to Cozumel to catch the morning dive trip to some of the world's best reefs, with a Cozumel dive operator. You'll want reservations in advance, and most shops leave for the reefs between 8:30 and 9:00 for a two tank dive in the park. Travel from the mainland to dive Cozumel can be difficult to keep the schedule of the people staying on the island.
You're in the middle of fabulous reef's so don't laugh. . . try the fresh water Cenotes! Water flows underground in the Yucutan. . . You'll need a local guide, some education in cavern diving, usually part of a days diving. And some thermal protection to dive the Cenotes. Spectacular visibility, Stalactites and stalagmites, it's addictive. . . some people have turned their back on coral reefs to dive these caverns, and for the better trained, even learned to cave dive. Again look under the "Mexico" threads for past recommendations.
There is enjoyable diving on the mainland, so enjoy it now that you're there. . . Next month when you travel back, stay on the island. . . Follow the path of Calypso, and Jacques Cousteau, while riding the currents past spectacular reefs. Their underwater park is protected by the local divers, and the fish and lobsters are easy to watch because they aren't afraid of divers. It's OK if you wait two months till your next trip. . . Cozumel is addictive and we're all Cozuholics.