Avoiding crowds on a ship (especially at full capacity as they seem to be running these days) is impossible. Wearing a mask at all times while indoors on a ship is not only extremely uncomfortable, it also interferes with the whole cruising experience. I understand why someone would wear a mask if they are at high risk but there are also other vacation alternatives that are far less crowded and just as enjoyable.
The pandemics that have become endemic (like Covid) or completely eradicated such as the Asian flu, the swine flu, Spanish flu, SARS, MERS, H1N1 etc., etc. are much the result of effective vaccines and not face masks. But if one needs double protection, why not? To each their own.