I really recommend you to snorkel in Akumal Bay. Just head for restaurant Lol Ha and you are going to see the beach from where you can make good snorkeling. You will probably see other people snorkel as well but there is still enough lots of space for you. If you´re going to snorkel just one day I think you should rent your gear. I planned to snorkel more than once so I bought my own equipment at home and brought it with me.
Here´s a few pics from Akumal Bay http://www.playa.info/playa-del-carm...tml#post756490