In addition to everything above, I would add that, if possible, line up at the exit that will get you off the ferry quickly as the time to process people can take awhile. Its not always easy to guess if they're docking on the port or starboard side as the boat will sometimes do a 180 degree turn at the last minute for its next departure. At this point people are in a hurry to get ashore and there can be some pushing and shoving.

Taxis don't leave the STT airport until they have a full load so when you land, if you're in a hurry, find a taxi that is almost full of people.

Have a pen in your pocket to fill out the Customs form (you'll get the form when you buy your ferry ticket). Your "arrival vessel/aircraft" is the ferry you just rode on, NOT your Airline & Flight Number that got you to STT.

C&I will give you a copy of the form you filled out. Keep it as you will need it when you leave the islands.

Checked luggage sometimes get stored on the bow. In a rough crossing, they will get soaked. Holding onto your bags keeps them dry and makes for a speedier exit from the terminal. If you have to check your bags, you clear customs and then pick up your bag before you leave the Roadtown Terminal. They also have x-ray scanners (similar to an airport) that you and your bags go through when you exit as well.

Don