I have snorkeled there a few times. My favorite place is Palancar beach where you get a boat out to the Palancar reef for about $25 pp. They take you to shallow and also deep (30 ft) places. However, if it is windy, this may not be a good place. Once we hit it when it was windy and went to Chankanaab which was sheltered from the strong wind. Chankannaab is a nice place to snorkel too.
Hope this helps