IF you are eligible for a passport it is the best option for the following reason. Say for instance you for some reason miss the ship during on of the ports of call, you can atleast get on a flight with little to no resistance. Passports are running a bit pricey ($165) and can be obtained at many larger US postal service locations.
A great alternative to the passport book which most people have, is the passport card. The difference between the two is the the card (which is the size of a standard issue drivers license) alot cheaper (about $55) and the card allows you to use it as a passport for both land and sea travel (IE Cruises or Ports of entry on the Canada and Mexico border). You CANNOT travel internationally by air with this though, something to consider.
You can also get expedited service if you need it in a hurry, for an additional cost of course. All this information can be found with a quick search with your favorite web browsing service or by going directly to the State Depts website.
"I've seen Monkey Poo fights at the Zoo more organized than this place!"...............