The definition of being 'fully vaccinated' will depend on the cruise line's protocols and the country you are travelling to. Please check your cruise line's requirements and visit Foreign Travel Advice .
In most cases, the definition of ‘fully vaccinated’ is having completed an approved (by the Medicines and Healthcare products Regulatory Agency (MHRA) or the World Health Organization’s Emergency Use Listing (WHO EUL) two-dose COVID-19 vaccination course, or the approved single-dose Janssen COVID-19 vaccine, at least 14 days prior to travel. If more than 270 days (calculated from the last day of the cruise) have passed since the full completion of a vaccination course, then a booster vaccine will also be required and must be administered a minimum of seven days prior to embarkation. Regardless of the date, if you have already had a booster vaccination then you are eligible to travel.