This was our second land/sea and we did much better this time. What we didn't realize the first time was that by having a PO Box as our mailing address they wouldn't send the luggage tags for Magical express! So this time we did it right, walked right off the plane onto the ME bus.
Once at the hotel (we did hotel then cruise) they give you the key to the world card for the entire stay. They then ask you stop by the cruise desk when you get a chance. When you do they will take a look at your cruise docs, take them from you and you are done! Checked in for the entire thing! It is wonderful because when you reach the cruise terminal you just walk right into the ship - no lines!!!
Another nice thing is that you actually have park admission for 7 days - so thats why we do the extra 2 nights afterwards - so we can use that all up. Also on the Sunday you leave for the cruise, you have a choice, you can leave from the resort at about 11:45 or leave from AK at 12:45 - so if you want you can even go to AK that day, have a character breakfast or whatever! (thats if you do their transfers - which I actually found very easy and convenient) - I know you are doing a sedan but thought I would throw that in.
Our land charges were settled when we checked out - but it was done via Express check out since we had a cc on it.
Castaway Caye was quite nice - definitely shorts weather - a little cloudy but I loved it. The water was a little chilly for dh but I think that just depends on your taste - he likes it 80 or above

This time of year it was not crowded - in fact very nice number of people on the cruise.
Let me know what other ? you have - I am glad to help!!
