Dear Silke,
As of today, there is no Covid restrictions entering Hong Kong. No vaccination or testing requirements and no quarantine.
You are free to roam the city. No vaccine pass required.
Covid-related boarding requirement is up to individual cruise lines I believe, not by the government.
(P.S. As an incentive to visitors, the HK is giving away 500,000 free air tickets to visitors via the airlines. If you are interested: https://www.hongkongairport.com/iwov-resources/html/wow/tickets/en/)