Penny is spot on here... There are many taxi's waiting at the end of the cruise terminal mall (before you get out to the "real island" Talk to a couple and ask what they would charge to give you a tour. For reference, last June, one took us to Red Hook (not a short trip from the port) for about $30. (Family of $4) It was raining so he proably charged us more than we should have paid but I wasn't going to stick with plan A which was taking the public bus.
But it is a relatively safe Island and the locals were friendly. I had no problems taking the public bus ride back from Red Hook all the way around the island back to the ship for $2 each.
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, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013