Well, try to take the ferry from Charl. Amile (where your ship docks) to St. John, avoid taking a taxi to a closer ferry or it will cost you about an extra 40 dollars cause of cab fare.
What line are you cruising on? What ship? How long do you have in port?
Also, when you get to St John, a cab will take you to Truk bay, they will stop on a hill to give you a great view make sure you take a lot of pictures.