Morning Judith
I think if he he down as a Master Mariner, then you are safe to say he was in the M.N. Have spent the last year researching my six M.M.s.
To me it seems that you have three main stages :
1. If you or your library have access to Ancestors.com - go to their Marine page and search for his Master's Certificate - at his EPOCH this will be a service cert (usually given around 1850 -make sure you take a copy of reverse side, as this will give you his sea service and ships up to that point.
2. From then on the ships that he sailed on and where they went ( in general) can be found on the Captain's registers at L.M.A ( if you are able to get to London) or you can write to The London Metropolitan Archives : ask.lma@cityoflondon.gov.uk/lma I have always found them very helpful, oh yes, and ask them to send you : Info Leaflet Number 50 "Lloyd's of London 'Captains Registers' and related sources". On this subject, note on page 2. the reasons why you might not be able to find your Captain on the register - couldn't find one of mine as he had died in 1868.
3. Having got your ships, trawling through Lloyd's List & registers will give you even more info, and of course not forgetting the Newspaper Archives.
Best
John
p.s. Yes there are rather a lot of Wilkinsons, have also a cross ref with a Mabel Joplin (a draper's daughter) but note without the'g'.