As os Saturday 24th September, Greece removed their requirements for cruise ship passengers to test prior to boarding.
Celebrity have updated their protocols now to reflect this, however Royal Caribbean don't appear to be yet.
https://www.celebritycruises.com/healthy-at-sea/italy-travel-requirements
"Please Note: Greece has removed its unique testing requirements as of Sept. 24, 2022. Please disregard any previously received communication with special instructions for sailings visiting Greece and follow the above guidelines."