Have done it with less time many times. There is usually a later ferry also. Have never been late in all these years coming into STT. Baggage comes off quickly and just head outside there will be ton of people looking to give you a ride. Just look for a van and you are set. Its a 15 minute ride to the ferry dock. Walk up to the window and ask for the next ferry. Pretty straight forward. Ferries tend to leave late anyways due to them bringing mail and packages over. Do yourself a favor and check in and go upstairs to the Petite Pump Room and grab a Pain Killer for the beautiful ride over to Road Town. Have a great vacation.