It's always better to do your own air because it's alot cheaper.
Disembarkment (I mean when they actually let the people off the ship)usually begans somewhere around 8AM. Sometimes it can be earlier or later it all depends on customs. At some point in time when you get on the ship(it doesn't have to be right away), go to the Pursars office and let them know your situation and inform them they you must be among the first to get off. You may be asked to show the ticket so have it available.
Also, you suppose to supply the cruise ship personnel ahead of time of your travel arrangements. So do that and just serve a reminder when you board.