Crew lists of the British Merchant Navy from 1915 have been digitized and made available for free online. Fully 750,000 names from 39,000 crew lists were scanned. (Those numbers alone indicate the sheer size of this service.) You can search by name, vessel, or rank; when you come across an entry, just click on it and you will be directed to the appropriate list online. Other online directories of mariners from Ireland to Australia are also available.
Online Directories of Merchant Seamen