Yes, you can book when you get onboard, but I would recommend taking another look at days/times, etc. and see if you can't find something you can fit in. You can also go to any one and wait in the stand-by line and they will let you in, 10 minutes before the performance to claim any remaining seats. We actually did that a few times and were able to find seats, but there were not that many left. There are also computers throughout the ship you can check on seating, but I highly doubt it will change from what it is now... Just keep checking and get a slot that will work and manage your schedule according to that. Good luck and enjoy the amazing entertainment.