Sounds like you have plenty of time. We spent as much time as we wanted at Trunk bay and still had enough time to go back to the ship, shower, change and go back out for about an hour of shopping... The bus stop is not easy to find unless you have done it before. Get outside the port area and ask a local. Its not too far from the entrance to the port. I actually enjoyed the bus ride. We got to see the "real life" of the island which was nice. Hope you have fun!
Also, when you get to St John's check the schedule for the returning Ferry. Then clock the taxi ride time tot he bay and you will know when you have to leave to get back in time to catch a ferry without having to wait too long.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013