You can't walk into a cruise port and book a cabin for that morning’s cruise. After 9/11, cruise lines have to submit their passengers manifest to homeland security 72 hours prior to the actual cruise. This means if you are looking for last minute cruise deal then you need to reserve it before this 3 day deadline. Since there cannot be more passengers than the lifeboats can hold, there sometimes are situations of empty cabins that will remain empty fo the duration of the cruise. This happens when there are too many triple occupancy cabins.
What should you know when looking for unsold cabins.
By waiting to book a last minute cruise by looking for cabins that have not been sold, you could easily end up with the stateroom in a noisy area or a highly undesirable location like in the rear where you hear the engines. The best located cabins are usually the ones that get booked first. On most cruises there are a few cabins available just in case something happens to one of the cabins and they need to move passengers to a new room.