Thanks to the PYA for providing this information.

A seaman’s book is  is a continuous record of a seaman’s service. This document certifies that the person holding is a seaman (as per The International Convention on Standards of Training, Certification and Watch keeping for Seafarers (STCW), 1978, as amended) and that he/she is working on a yacht. It can also serve as a passport of sorts. It is  one of the compulsory documents for applying crew transit visas.

The difference with the testimonial is that in the seaman’s book you will only have a record of the date signed on and the date you signed off the yacht. No days at sea are mentioned or shipyard days etc or other details…

CHEF Testimonial example click here