Go to Red hook on STT, take the ferry, which leaves every hour on the hour untill very late at night, to STJ. It will set you back 3$ each way per person. Once you are on STJ, you can rent a jeep for 50$ a day or if you only want to go to one or two beaches, you can take a taxi to and from the main beaches(trunk, hawksnest, cinnamon) because the taxis are all around. My opinion, if you want a great snorkling experience, is go get your rental jeep and head to the other side of the island towards coral bay. Get directions to Salt Pond Bay. It is hot down there, so bring water. You hike a third of a mile to the beach and there is a whole bay to snorkle. You will see some of the best stuff, maybe rays, turtles, big fish, conch, and all types of coral. Then hike back and eat lunch at skinny legs....rest up and then if you still have daylight and energy, you can go to trunk which is rated as the one of the worlds best beaches(but not snorkling).....and or Hawksnest and cinnamon or any number of beaches on the north side of STJ. To me, I think the beaches on STJ are better than only one day of exploration, considering STT beaches arent to much to hollar about.
Lunch up top of bordeaux mountain is pretty fun too...such a great view.
Have a good trip.