They have the underwater sculpture garden which can be a snorkel or a scuba. Also there are several dive shops on the island that list a dozen or so dive sites around the island. The dives tend to be deeper than those in nearby Cancun.
I have only dove a few sights as I am going there mostly for whale sharks. I charter a small boat with one of the best operators based on the island. I have 6 guests on the boat and we go for 4 days with the whale sharks. Sometimes we arrange an afternoon dive somewhere around Isla Mujeres, Mostly the guests extend their trips for diving in Cancun or the cenotes. The cenotes are very nice and work doing (Playa del Carmen)
If you want to see my whale shark trip:
Swim with the Whale Sharks of Mexico - Limit of 6 Guests on our Private Boat