I agree with those who say it's wise to go ahead and get the passport. Although the requirement for our cruise was that you would ONLY need the passport if you had to fly home, we did not want to take even that chance. All five of us are over 50 and have an elderly Mother who stayed home. I was concerned that we might HAVE to fly home and would not be able.
The passport requirement is something that has been needed for a long while and although it was expensive and time consuming, we are so glad now that we have them.
I even gave a friend's son his passport for a graduation gift this last summer (paid the fee). Now if he and his college buddies decide to go on Spring Break on limited funds, he won't have to worry about that added expense or the last minute application.