This is from P&O website
ETIAS Update
In the near future, British nationals travelling to the EU will be required to obtain an ETIAS travel authorisation prior to sailing with P&O Cruises for all itineraries that include an EU port. This travel authorisation is a new EU regulation and will be proactively enforced. As with all visa and passport requirements, applying for the ETIAS travel authorisation is the responsibility of the travelling guest. The current time frame for this new regulation to come into effect is May 2025, and for any guests sailing to the EU in 2025, further information will be communicated in due course as it becomes available.
For more information please visit - https://travel-europe.europa.eu/etias_en
However, that link does not appear to work!!