STJ is not shutdown in September and busier than most of the BVIs. The kids are in school and many restaurants stay open. Outside of tropical storms, the water clarity is great, the wind is light, and it can get pretty steamy. Coral Bay will be very quiet with Skinny Legs closed, but Surf Club Cantina and Sharky's will likely be open. For boat charters, you'll need to check with the operators. Some shelter in Hurricane Hole, some trailer their boats, many sail charters will be at Carriacou or Grenada.
Cheers, RickG