I noticed you hope to both shop and get the boat checkout done on your arrival date. You state you are taking the fast ferry. That will have you arriving in Roadtown about 5pm. By the time you clear customs and cab to your boat it’s going to be 6:30 or later. Since your flight arrives at 1pm I would try and catch the native son ferry at 2:30. Customs should be quicker on the earlier boat also. Either way don’t plan on doing to much other than having a nice dinner on arrival.