This is their website http://gbferries.com/ - they currently leave from the Simpson Bay bridge/police station (as do all the shuttles). I have heard recently that it is more efficient (and cheaper) to take the public ferry from Marigot.
All public ferry and private shuttle info can be found on the following link (pdf):
http://www.carimar.com/carimar/updated-flight-information-post-hurricane-irma/
Happy planning.
Ellen

Life's a beach