As has been mentioned you need to be out of your cabin early, and most people will just go ahead and get off the ship then so it gets busy. But what I like to do on days I want to get off later is head up to Lido and enjoy breakfast, and debark when I want - on a cruise last year we had a late flight so we stayed on until last call. You can pretty much debark when you want if you are handling your own luggage.