We did the 2:30 out of STT just after New Years, from JVD. 9:15 Inter-Island ferry left Great Harbor, stopped briefly in West End, continued on to STJ for customs/immigration, and dropped us in Red Hook. We caught a taxi to the downtown ferry dock and stowed our bags for an hour while we had lunch at the Petite Pump Room. Then we headed to the airport with time to spare. It's definitely doable (if your ferry runs on schedule).

Felix