Definitely take the 9am ferry. Doing BVI to STT via ferry is a big PIA. You have to go through customs/immigration as you disembark the ferry and then you get to do it again at the airport!
American Airlines is notorious for long lines and their self service kiosks rarely work. If you are checking bags, you have to wait in their lines to get your bag tags, even though you bring your bags with you through customs/immigration. If you are not checking bags you can use a mobile boarding pass which will save you time. The next line you will get in is to go through customs/immigration. That line can wrap around the entire front of the airport on a busy day which is typically weekends between 1:30-4:30 but it can be any day at any time depending on the departure schedule. If you have Global Entry you can skip this line and go to the front of the line where the porters go in. You can also skip this line if you get a porter (red cap). Once you go through customs/immigration you get in the line to drop your bags. If you don't have any you can go right to the TSA line. Precheck helps some here but honestly not a ton. There is a restaurant and bar inside the terminal and you can get decent food there and cold beverages.