Tulum sells two kinds of water. The Caribbean is on one side of the road; on the other, under the jungle, is the longest surveyed underwater cave system on earth, and the cenotes are the holes where its roof fell in.
Cenote diving is the reason most divers come. It is not reef diving: the water is fresh, cold enough that you want a thicker wetsuit than the sea needs, and so clear that the light shafts coming through the collapse look solid. A guide is required by law, and the distinction between a cavern dive — always within sight of daylight, open to any certified diver — and a cave dive, which needs its own certification and is where people who ignored the rule have died, is not a technicality. Dos Ojos and Casa Cenote are the gentle introductions; The Pit, Angelita and Nohoch are deeper and darker.
The reef off Tulum is quieter and less visited than Playa's or Cozumel's. From November to March, bull sharks gather off Playa del Carmen an hour north, and most Tulum shops run the trip. Whale sharks are a summer thing and happen off Isla Mujeres and Holbox, three hours away; shops here sell the day out.
Snorkelling gets you into several cenotes without a certification, and into the lagoons of Sian Ka'an to the south.