I have heard there is a caves snorkeling excursion but have not done it. The beach is off of the Malecon, and while the water is nice, it's not like Cozumel. Most take a bus into Merida or go and visit the ruins. Chichen Itza, Uxmal, X'Cambo, and Dzibilchitun. Chichen Itza is probably the most notable. I don't remember a good snorkeling tour offered when I was there, but it's been many moons ago