Merchant seafarers are well documented compared with other professions. Most of the records are held at the National Archives although to complicate matters the documentation changes over time as each system set up by the Board of Trade was overwhelmed by the growth of Britain's merchant fleet. The Archives Centre at the Maritime Museum has an information sheet that explains how to track the records down.
This cartoon above is taken from the wartime log kept by merchant seafarer Peter Rogan while he was a POW in Milag Nord during the Second World War. With so much merchant navy history to cover, this is just a small example of the service given and hardship suffered by merchant seafarers, as documented in the Archives Centre.
Mariner's Register Ticket, issued to Thomas Crone, reference DX/850
This image is of a Mariner's Register Ticket, issued to Thomas Crone born Liverpool, 1833. The seafarer was required to keep this document and give it to the ship's master when he signed the crew agreement. The other half of the tickets, which were issued to seafarers between 1845-1854, are held at the National Archives and list the voyages of the seafarer.