I think I have read on other boards that you must have a passport for this cruise since it goes to Fanning Island which is another country. Suggest you check with your travel agent or cruise line.
All US citizens MUST have a valid US Passport in their posession for this cruise as it calls on Fanning ISland in the Republic of Kiribati. You will be denied boarding if you show up with just a birth certificate.
You do not need passport to go to Fanning Island, but the United States Immigration Service requires you to have a passport to return to the United States of America.
Since when does a US citizen have to have a passport to get back into their own country? A birth certificate will do. Having been to 31 countries....WITHOUT a passport I think you are incorrect.