John McKenzie/Jemima Junor

[Maxn]

I have previously received assistance in researching the above names who were married Rosemarkie, Ross and Cromarty 29/08/1838 and are known to have had at least 1 son Alexander, born 22/3/1841.
John, the father was said to have been a Mariner, who at the time of the 1841 Census was said to have been at sea. Alexander and his mother Jemima, were living in High Street at the time. She was recorded under her maiden name of Junor.
I have unfortunately come to dead end on my research into the parents of Alexander (my G.G. Grandfather) and would welcome any assistance in being able to trace them further.
Alexander came to Australia abt 1867 ( this approx from Marriage/Death certificates) but as mentioned his parents remain a mystery.
If John was at sea in 1841 is it possible to determine where? Was a mariner a term given to just anyone who went to sea , a fisherman for instance or perhaps a merchant marine?
Any ideas?
Regards, Maxn