Something I didn't state earlier as to why you have to book in advance, either at Guest Relations or when you book your cruise. The cruise line will have a list of approved passengers that have purchased transfers to Orlando Airport. You must either give your name to the representative, who will check you off the list, or in my case, hand them the "post cruise voucher" which I received with my docs since I purchased transfers when I booked the cruise.
I have to assume they're pretty strict about this, and if you're not on the list, you probably won't be allowed to board the bus.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)