Although the Merchant Navy is a civilian service, during periods of ‘hostilities’, some seamen served in the Royal Naval Reserve.  

Two useful publications are:

Records of Merchant Shipping and Seamen by Kelvin Smith, Christopher T Watts and Michael J Watts (Richmond, Surrey: PRO Publications, 1998)

My Ancestor was a Merchant Seaman: How can I find out more about him? by Christopher T and Michael J Watts (London: Society of Genealogists Enterprises, 2004).

Most First and Second World War Merchant Marine records have now been passed to The National Archives from the Registry of Shipping and Seamen.  The National Archives provides helpful information sheets explaining what records are available at http://www.catalogue.nationalarchives.gov.uk/researchguidesindex.asp#m

You can consult the Second World War Merchant Navy campaign medal roll on The National Archives website. As medals had to be claimed and were not automatically issued, if you are unable to find your relative listed on this it is probably because their medals were never requested.

Excellent sources of information for the Merchant Navy can be found at:

Guildhall Library
Lloyd’s Marine Collection
Aldermanbury
London EC2P 2EJ
Tel: 020 7332 1868
The Guildhall Library website

The National Maritime Museum
Romney Road
Greenwich
London SE10 9NF
Tel: 020 8858 4422
Website: www.nmm.ac.uk
Information sheets: www.nmm.ac.uk/server/show/nav.005002006000003

Most Crew List and Agreements are held by the Maritime History Archive, Memorial University of Newfoundland, St John’s, Newfoundland, A1C 5S7, Canada.  Their website address is www.mun.ca/mha/  The National Archives and the National Maritime Museum also have some, but guidance about this is given on their websites.

The Imperial War Museum has some material that can help with Merchant Navy research, for example, a selection of Lloyd’s Register of Shipping for the First and Second World Wars. 

Books that provide details of ship losses are listed below:

Dictionary of disasters at sea during the age of steam including sailing ships and ships of war lost in action, 1824-1962: [2 volumes] by Charles Hocking (London: Lloyds Register of Shipping, 1969)

Lloyd's war losses: the First World War: casualties to shipping through enemy causes, 1914-1918 (London: Lloyd's of London Press, 1990)  ISBN 1-85044-314-9

Lloyd's war losses: the Second World War, 3 September 1939 – 14 August 1945: [volumes I and II] (London : Lloyd's of London Press, 1989-1991) ISBN 1-85044-217-7. Volume I: British, Allied and Neutral Merchant vessels sunk or destroyed by war causes.  ISBN 1-85044-412-9. Volume II: Statistics showing monthly losses of British, Allied and Neutral Merchant vessels

Shipping line archives are also good sources of information:

Company archives the National Maritime Museum can be found at http://www.nmm.ac.uk/server/show/conWebDoc.599

The Merseyside Maritime Museum also has listings of their collections at http://www.liverpoolmuseums.org.uk/maritime/archive/majorcollections.asp

Company archives will be found in different repositories around the country – if the shipping line you looking for is not listed on either of these sites, try this link http://www.nationalarchives.gov.uk/searchotherarchives/

You may also wish to contact

The Merchant Navy Association
National Office
9 Saxon Way
Caistor
Market Rasen
Lincolnshire
LN7 6SG
Tel: 01472 851130
Website: www.mna.org.uk