In 1747 an Act for the Relief of Disabled seamen was introduced which, among other things required the masters or owners of merchant ships to file Crew Lists with the Seamans Fund recievers at the ports of sailing and arrival.
Few of these crew list remain from the very beggining adn those that do survive only exist for a limited number of ports (Sheilds, Plymouth, Dartmouth, Liverpool). Most crew lists from about 1839 exist and these can be a good source of information.
Typically the crew list will include the name of the Ship, its port of registry, where and when the seaman joined the ship, Voyages undertaken and his Job title.
For more information please see…
My Ancestor was a Merchant Seaman: How Can I find out more about him?
CT Watts, MJ Watts, 1987, The society of Genealogists