Possible Explanation?
Watching a video on Virgin Cruiselines, it was explained that passports with the six month rule were required, close looped or not, as their ships are flagged under the British flag.
‘This would make sense that it is the country under which the ship is flagged that determines the passport rules for the ship, not the country of the passengers or the country of the embarkation.
‘Oceania ships are flagged under the Marshall Islands and the passport rules are six month expiry, close looped or not.
Make sense?