You can search the Registers of Seamen's Services online for those who entered the service from 1873:
http://www.nationalarchives.gov.uk/documentsonline/power-search.asp?searchType=powersearch&query=&catID=15&mediaArray=*&pageNumber=1&queryType=1

If a man entered before 1873 he may only appear in the series ADM 139, which is not online.

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