You will need to get a us customs sticker in advance for your boat as I doubt moorings put one on it. If you do this the app generally works well. Otherwise take a mooring at Lind Point and Dinghy into US customs. You can clear out at either Jost or Sopers hole. All of the national parks bays have moorings. It is illegal to anchor in the park.

This national park map is very useful for visitors. St John National Park Marine resource guide