Great review! We will be boarding the Jewel for the China/Japan cruise shortly. We are currently traveling from Bangkok to Hong Kong.
Regarding passports at the Grand Palace. We were told by our hotel that passports were required; they were not. The only reason you would need your passport would be to rent an audio guide. Passports were not checked at the ticket booth.
As for passports being collected on board, you can go to Guest Services to retrieve your passport at any time. They are collected to make immigration easier; all of that is taken care of by Guest Services saving you time while in port. However, you can get your passport back if you need it for a particular port.
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