For Merchant Seamen, if he served after 1918, there is an index to the record cards which are at Southampton Archives:
https://www.southampton.gov.uk/libraries-museums/local-family-history/southampton-archives/index-merchant-seamen.aspxYou can also search an index of Merchant seamen here, but it won't give as much information as the original cards would.
https://familysearch.org/search/collection/1762440
Presuming that the 'Pritchard' name he didn't pull out of thin air, this man might be of interest.
Henry Pritchard, b. 15 Dec 1881, Poplar, signed up to the Royal Navy 1897:
http://discovery.nationalarchives.gov.uk/details/r/D6702726There is no Henry Pritchard birth registration that matches up with that birthdate, as far as I can see (should be registered Dec 1881 or Mar 1882 quarters) - possible he changed his birthdate or didn't know his true birthplace or birthname - often young boys who went into the Navy did so from industrial schools and the like.
Also possibly of interest, this boy in 1891, Harry Pritchard aged 10 living with grandparents:
https://familysearch.org/ark:/61903/1:1:4QTR-7T2He is not the "Henry James Pritchard" born 1880 in that area - that's the son of James Pritchard b. 1847 and is with his family in the same census.
Following the family back in time shows they had many children including a 'Kate Pritchard'
https://familysearch.org/ark:/61903/1:1:QKX9-418Zhttps://familysearch.org/ark:/61903/1:1:VFDB-VL7James Pritchard married Elizabeth Blake in 1863. He was married before, so some of the children seen 1871 onwards are his with his first wife:
https://familysearch.org/ark:/61903/1:1:M7C3-44ZBut "Kate" isn't - possibly she was born Blake and was the mother of "Kate Blake" listed as a granddaughter in 1891?
I think "Kate", the younger in 1891 may have been born "Florence Kate S Blake" in 1880. She's not the "Florence K S Pritchard" who married in the area in 1921 - that's a woman born 1895.
It's possible that "Harry Pritchard" in 1891 is the Harry Edward Pritchard born March 1881 Whitechapel district - he appears in the Eling area in 1901:
https://familysearch.org/ark:/61903/1:1:XSXN-G1YHe is listed as 'boarder' here but "Amelia Abbott" is nee Pritchard so is his aunt if he's from the same family. He seems to disappear after 1901, or at least I can't find him. Neither can I spot possible sister/cousin Kate/Florence Kate.