Our favorite place to snorkel is Boca Catalina Beach. If you head northwest toward the Lighthouse on Jose Irausquin Blvd., you'll come to it between the Marriott complex and Arashi Beach. (Arashi isn't bad either!). Catalina is a nice little beach with great sand and coral very close to shore. There are some decent size reefs there with a good variety of fish.
Catalina doesn't have a marked parking area like some of the other beaches, but there are spots to pull off and park.
If you want a cheap, friendly and quality snorkel cruise, we'd highly recommend Roberto's Water Sports. They have a stand in front of Moomba by the Marriotts. He offers a 3 hour trimaran snorkel trip with sandwiches and free drinks for $35. They take you to Catalina and the Antilla wreck.
http://www.robertoswatersports.com/
They also do private snorkeling trips. Enjoy!
Enjoy.