WE did the Shark/Ray ally trip from Carnival Cruise a couple of years ago.
If the one you take is similar, the snorkeling is terrific on the tour. On the one we took, after the snorkeling, they brought us to Wet Willies where lunch was included. We then had about an hour to tour San Pedro. Which was nowhere near enough time to see the town. (therefor, we are coming bach for a week this April)
If I were you, I would enjoy the snorkelling that they provide from the boat and spend your time in San Pedro expoloring the town rather that trying to snorkel from shore.