Consider Wharfside Watersports for everything from snorkel gear to dinghy/zodiac rentals to captained charter boats. Their retail shop is in Wharfside Village on the beach in Cruz Bay. Take a right off the people ferry dock, walk past High Tide and Bamboo jewelry and find them behind The Rum Hut.
You can even pre-order your snorkel gear rental sets and pick them up while the rental cars are sorted out so you will be ready to swim with some turtles on your first morning.
Make sure to mention you are staying at Great Expectations!