There is a card index BT 350 (CR 10) available on microfilm at The National Archives. These cards often include a photograph, and are arranged in alphabetical order.

Merchant seamen: Registers of service 1913-1972

Merchant seamen: Medals and honours

My Ancestor was a Merchant Seaman, Second Edition, Christopher T. Watts and Michael J. Watts (Society of Genealogists)