I travel nearly week of the year so I am probably willing to take more chances with air travel than most folks but I don't think you will have any trouble with the 10 AM ferry. In fact, I have taken the 12:15 ferry a couple of times and still made a 2:30 out of St. Thomas. I wouldn't recommend cutting it that close but I would do it in a heartbeat if it meant the difference between spending the last night at Norman Island versus the base. If it gets close you close always pay a porter to take you to the front of the customs line.