When are you cruising?
If it isn't until late June or July, why not go ahead and get the passport. It would eliminate any problems. Everyone will have to have one to cruise the Caribbean after 12/31/05.
If you do miss the ship or leave the ship (permanently) in ports such as Barbados or Aruba and do not have a passport your life will become very complicated.
Until 12/31/05 you will be able to sail with a birth certificate and government issued id, provided you are a US citizen. If not additional documentation will be required.