Hi Jenni
Websites which might be useful are
www.mariners-l.co.uk which is the Website of the Mariners Mailing List. Also a great many crew records are kept in the Maritime History Archive in Newfoundland, at the Memorial University in St. John's. Their website is
www.mun.ca/mha - they have a list of principal records of British shipping and seamen showing their present locations. Some are in the PRO at Kew but it depends which years they have kept. There is also a lot of information on the website for the Greenwich Maritime Museum
www.port.nmm.ac.uk.
The Merchant Shipping Act of 1835 required masters of all ships to file Agreements and Crew Lists with the Registrar General of Shipping and Seamen. The crew signed this on joining and again when they left and collected their pay. There is a signature on the front of my copies which gives the names of the master - I assume this is the captain?
If you know which shipping line he worked for you might be able to find out where the records are. I have researched my grandfather who worked as a master baker on Union Castle and P & O liners, operating from England, and all these records were in Newfoundland. They will research and do copies for a fee - all information on the website or you can write.
Good luck!
Honeybun