To work on a cruise ship, you must meet the following basic requirements:
Be at least 18 or 21 years old.
Meet the language fluency requirements.
Job related work Experience minimum 2 years plus. 
Have a valid passport & Seaman book.
Physically and mentally Healthy.
Have a basic safety training certificate STCW-10
Pass a medical examination.
Pass a criminal background check – police Reports.
Some academic and professionals’ courses.

What is Seaman book?
Seaman's book is also known as seaman's services book or seaman's record book, which is a Continuous discharge Certification (CDC), a continuous or it is a Seaman Travel passport. Without seaman book no one will not get employments in the shipping lines.  

What is needed to apply
STCW-10 – five courses certificates.
Copy of Valid signed passport (ID Page)
Current medical OEUK/OGUK or the particular medical form for the flag state needed. You can find these Medical forms located at the bottom of the appropriate page.
Passport size photo.
Completed and signed application for the appropriate flag state.
You will be required to pay a seaman's book fee, which varies from country to country. In most cases, it consists in the submission of an application form, along with valid documents such as a valid passport, photos, medical certificates and other administrative requirements.