Freedom - I've had ferry stress in the past. You might consider reserving Dolphin Water Taxi. It's expensive - $140 per person - but they pick you up at STT, they'll wait for you if your flight is delayed, they drive you to their dock on the east end of St. Thomas, and they can deliver you directly to your charter base and handle your customs requirements. You're not bound to the ferry schedule, don't have to deal with lines and people pushing and shoving to get aboard or luggage winding up in the wrong place (that happened to me too).

Just a thought.