The cruise may either require a Covid test to be taken that is medically observed or a self-test. These will need to be purchased from an independent supplier and can be arranged to be delivered to you or an appointment booked to do in person. Medically observed tests may also be available to take from home via video conferencing but it will depend upon the supplier. Instructions come with all kits and documents will be emailed to you proving your negative result. You will need to print these out and take them with you as you embark.
In some instances, cruise lines may not require proof of a negative test but will request that you self-test before you board, these can be purchased from a pharmacy. A declaration may be required to sign and random tests may occur at the port.