Didn't the couple ignore the same warnings? It's not like they did not know the volcano was there
Did the cruise line receive an official warning from New Zealand that they didn’t vpass on to passengers? Or did everyone involved have the same info? If you have Japanese tourists in Cincinnati, I doubt they would have the same info on godless tonadoes attacking that the natives do (from a WKRP episode).
The following is a pure supposition but it is how I can see the problem occurring.
Frequently, one signs up for an excursion before the cruise starts and pays the extra cost in advance. This is logical as the company needs a valid number to make the excursion financially viable. Yes, there will be legalese about dangers and potential cancellations.
Come the day of the excursion and the passengers are prepaid with no refund if the excursion is still on. If the excursion operator has ignored warnings and the plaintiff lawyers can convincingly argue fault and negligence by both operator AND Royal Caribbean with a court victory, then that is the case. Royal Caribbean is the deeper pocket but may be harder to find at fault.
So let me leave this dangerous site to the decision of a cruise line. Where is the individual responsibility