When you check in at the terminal there is a special desk for non American citizens, that is where you go to check in, unless you are a
Castaway Club Member and then you can go to that desk.
When you check in they will take your passport or other ID away from you and give you a receipt for them. I am not sure why they have to do this but they do it.
Then for the rest of the trip you can come and go just with your Key to the World card, and a picture ID such as your drivers license or maybe health card with a picture on it.
On the morning of disembarkation you are required to report to the designated spot on the ship to clear customs, you have to bring your receipt for your passports with you then. Your passports are returned to you and you then have to report to the customs officer who asks you a few questions, (when do you plan on leaving the country etc.) After you are free to go back to your cabin and carry on with your day. I think maybe you have to fill out a form that is in your cabin and take it with you.
I don't know why just non-Americans have to do this because you are entering the US but American citizens have also left the country and are returning too?
This is quite long but i hope it helps, more questions just ask
Robin