Spend a day in sleepy Akumal. From Playa, you can rent a car, take a taxi, or take a colectivo (informal van system) to Akumal. If you are dropped off at the Arch, take a taxi to the Yal Ku lagoon as it is a long walk along Half Moon Bay. There is an entrance fee and you can rent snorkel equipment and a locker. The lagoon is wonderful for snorkeling. I prefer to go in via the smaller company which is the first one you come to.-- That way, you start snorkeling at the tip of the lagoon where the cenote was and the water is the clearest.
After snorkeling, have lunch at La Lunita for good food, or go farther to La Buena Vida for atmosphere.