Scuba Playa is a great shop in PDC. They offer full cave courses, etc. Great staff, really accommodating and professional. Erick was my OW and AOW instructor and he is absolutely fantastic.
I dove a handful of the cenotes around Tulum without having done a cavern course, just having my AOW cert and demonstrating good buoyancy control. Some of those cenotes near Tulum are not to be missed -- Angelita, Carwash and Calavera were all completely out-of-this-world stunning. It's a hike from PDC but they are hands-down some of the most incredible dives I've done. For that -- and if you want to get a cavern cert with easily the best tech and cave diver I've ever encountered, go see Nihls at Dream Diving in Tulum. They're on the main drag downtown, a small shop he owns with his wife. He is PHENOMENAL, and extremely reasonably priced as well.