How certain are you that William's father was a John? There is no William listed in the SP indexes 1835 to 1840 in the county of Lanark (I looked also in the RC indexes, but nothing came up there either). That doesn't mean, of course, that there wasn't such a birth. It only means there isn't a record of the birth/baptism.
Interestingly, there only one William Nicholson of around the right age in Govan in the 1841 Census returns. The first problem is that William's father had died before the census:
RD 646 Enum Dist 7 Page 1
Widow Jean Nicholson 25 [i.e., 25-29 - ages of those aged 15 and over were rounded down in the 1841 Census returns]
Agnes (spelled Agness by the enumerator) 10
John 7
William 5
also in the household were: John Freebairn 60 [60-64], a mason (perhaps Jean's father, though relationships were not given in the 1841 Census returns) and James Nicholson, 40 [40-44], a miller.
The only other problem is that this is the family of a
William Nicholson and Jean Freebairn who married 25 May 1829 in Paisley (District of Abbey) 25 May 1829 and had issue (all born within the parish of Barony, Lanarkshire)
Agnes b. 18 Oct 1829, chr. 6 Dec 1829
John b. 18 Oct 1832 chr. 18 Nov 1832
William b. 25 Jan 1835 chr. 15 Feb 1835
What you have to do is look at the original records on <a href="
http://www.scotlandspeople.gov.uk">ScotlandsPeople</a> and see from there whether the father was a mariner. If he was, then this
might be relevant for you. You would have to work out whether you were on the right track from other supporting evidence ... Was there confusion at the time of registering the marriage? Could the John actually have been Jean's father? (if the John Freebairn in these returns was indeed Jean's father). Or there could have been another mistake commonly found in the records - a misunderstanding about generations ... perhaps William's
grandfather was a John ..... One other avenue of research would be to look at all the marriages of a John Nicholson within a reasonable timespan (1815-1835 or so) anywhere in the country and see if any of them were mariners, but that could be inordinately expensive! They might have been a family that did not have their children christened (but, then, where was that William in 1841?). You also say "On the cert, his father (John) was quoted as a Mariner but was not deceased at the time". Actually, that doesn't necessarily mean that. It could well be that the registrar did not ask whether William's father was still living.
I also searched in England 1841 (just in case!), but there were simply too many entries to look at each one individually ... There was a William Nicolson, 5, son of John and Agnes Nicolson in Dunrossness in the Shetlands, but that John was a fisherman/farmer (crofter/fisherman), certainly not someone who would be described as a mariner.
