In my 10 or 11 trips flying into St. Thomas, we have never missed a same day ferry to Tortola. The first trip, our flight got to St. Thomas late. After claiming our bags, the guy in charge of pointing people to taxis knew the drill and a group of us were taxied off to Red Hook. The rest of the trips we ferried out of Charlotte Amelie (SP?).
Waiting for bags at STT airport probably averages out to 15 minutes. It just feels longer than that because of my anxiousness to get to the ferry so I can get to the boat and start my vacation.