Op depends on where you are staying and if you have a car. While the island is a grand total of 35 square miles, it can take an hour to get from point A to point B. (My 5.6 mile commute from Sapphire Beach to Charlotte Amalie took on average 40 minutes. More in bad weather, cruise ship days, accidents, errant cow in the road or pop up parade.) If you are on the East End/Red Hook area I can recommend Red Hook Dive Center. I also heard good things about Blue Water. Site is often dictated by weather and who is on the boat (students/haven't been in 10 years/. My favorite site is French Cap an 80 foot deep pinnacle. Mingo and Congo have a couple different sites, deeper more advanced. Calf and Cow are the iconic St. Thomas sites and I really do like Calf. Canyons and a surging swim through that pops you up like a cork. Packet Rock (named for a mail packet ship that sunk on it) has a china field, then a swim around the rock (I was stalked by a curious shark on a dive here). Fish Bowl or Aquarium has lots of fish. Dog Island has some interesting canyons to swim through. I saw sharks on about 30% of my dives, so keep your eyes open. Lots of sleepy nurse sharks and I'm guessing Caribean reef sharks often just at the edge of the visual field checking out the bubble blowers.