cruisemates, I have friends that have called up both Carnival and Royal Caribbean day of cruise in the last year and booked cruises, so it is definitely still possible. Usually more likely to happen in off seasons, like the fall for the Caribbean.

The only requirement is that they submit their manifests to the government 60 minutes before sailing. The only reason they ask for the information in advance is so that they have more time to prepare the manifest, but it is not required to provide the info in advance.