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Note: this is a practical question
This will be a business decision (price calculation) for each cruise liner to make.
Based on their experiences since March 2020, they must assume that the liner may become a 'hospital' ship and thus must calculate these costs into the fare.
Special agreements must also exist between the liner and the jurisdiction of the ports of call.
Some jurisdictions exist where the right exists not to ask for the vaccination status, but nevertheless would not allow (or cannot allow) infected persons into their jurisdiction.
So this catch22 situation must be resolved in some form.
So it is a matter of a legally binding contract between a cruise liner and their customers, together with the authority of the ports of call.
If all parties of the contract agree, there is no problem.