Riviera Maya, Yucatan
I recently visited the Yucatan to dive the cenotes for 3 days followed by 3 days looking for sailfish off Isla Mujeres. The cenotes are incredible dives. We visited some of the very popular cenotes (popular for good reason - they're beautiful) as well as some more advanced and off-the-beaten path cenotes. Bluewater arranged everything for us with diving through Scuba Playa, who I give an A++. Very professional and fun at the same time, with a nice van for transport and excellent guides who worked to help make sure we got the photos we wanted. They even carried gear for one of our group.
We also visited Isla Mujeres and spent three days looking for sailfish. These are natural, wild encounters, so you never know what you'll see. Over 2.5 days on the water we spend about 45 seconds watching the majestic fish corral and feed on bait fish. It's an amazing experience but you have to be willing to put in the time.
I gave the Yucatan only 3 stars for marine life because the marine life is very specialized. There is none in the cenotes, but that's obvious. Sailfish and whale sharks are special expeditions, while Cozumel has beautiful reefs and some big life (nurse sharks, eagle rays, turtles, good macro, etc).
Can't wait to go back!