It's not as crazy as it sounds. You could get "last minute deals" up to a couple of days before a ship sailed, but not the case now. The cruise lines must submit a list of passengers, crew, and cargo for security clearance. This is getting more stringent at every sailing we've done since 9/11. I'm really happy that they do this. We moved to Florida in 2000, and thought we could get some of the last minute deals, but they had even disappeared at that point. There's great deals out there. Go to a "cruise only" travel agent to book, and don't book over the internet.
Have fun if you decide on something.