When you check in ask if there is anyway of finding out if there have been any last minute cancellations, or you can ask them to notify you when everyone has boarded to see if there have been any cancellations or no-shows for the cruise.
Depending on availabilty you can upgrade your cabin with your booking agent prior to departure. Again depending on your cabin grade you may also be able to request a cabin upgrade upon check in.
Ask at the terminal as yo check in. we went in the school holidays and there was no availabilit.