Note also the Cunard site now clarifies
Cunard today have clarified this as well - in line with what Hattie said:
Guests who have had one booster vaccine, regardless of the date, are considered fully vaccinated. At this time, a second booster vaccination is not required, however, we strongly recommend all guests to have booster vaccinations at the earliest opportunity, should they be available to them, in order to comply with evolving vaccine policies from the countries we visit.