Vitran Bus System St John
Maybe the National Park Service can be of more help to your specific questions.
This is what they say on the above link:
"VITRAN buses travel between Cruz Bay and Salt Pond Bay (on the southeastern side of the Island) along Centerline Road (Route 10). The buses leave from the public ferry dock at 20 minutes past the hour beginning at 6 a.m. and ending at 7:25 p.m. daily."