A one-guide operation run by Rodrigo out of the La Toscana neighbourhood, north of Playa's tourist grid, diving the cenotes between Playa and Tulum.
The appeal is the size: a cenote with four divers in it is a different experience from a cenote with fourteen, and the cavern lines are narrow enough that group size genuinely changes what you see. Transport to the cenote is included; the shop is not a walk-in storefront, so arrange the pickup in advance.