Two shops, one business: Tulum for the cenotes and the coastal reef, and a second on 5a Avenida in Cozumel for the drift dives along the Palancar wall. Both open eight until seven.
From Tulum the regulars are Dos Ojos, Casa Cenote, Angelita and The Pit, plus cavern courses for divers who want to go past the daylight zone properly rather than by accident. The Cozumel side is where the current does the work and you mostly steer.
If you are writing to the island rather than the mainland, their Cozumel address is 5a Avenida between calles 2 and 4, on +52 984 169 3284 and [email protected].