I agree with Lisa, the best snorkeling we've done is Boca Catalina. There are good sized coral formations very close to shore and it's a quieter beach than most. Our next favorite is Arashi. You may also want to hop on a snorkeling tour. Our favorite is Roberto's Water Sports (booth near Hadicurari Restaurant on Palm Beach- link below). It's inexpensive and a nice trimaran ride to Boca Catalina and the shipwreck.
Yeah, I came across Roberto's yesterday. I think we'll probably try to make it to Boca Catalina and down to the rock formation by the Divi and Westin so they're easy to get to and free. We may give Roberto's a shot if we've got time, but I'm not sure how much snorkeling we'll have time for since we'll be scuba diving on most days.