Perhaps you have all the info you require however I cannot see John SWANSON born c1830 on the 1851 English or Scottish census other than John, the upholsterer born Portsmouth and living in Middlesex. I note that John and Margaret's marriage record shows him to be an upholsterer in 1853 when they married at Glasgow. And surely not a co-incidence John was an upholsterer in Fitzroy, Aus.
Donald SWANSON is shown as a Chelsea Pensioner on the 1851 census and witness for his dau's 1826 Catherine baptism which is a tad difficult to read, shows Donald's occupation as pensioner something about 79th Regiment. It may be the occupation of a witness. If Donald was in the army it may account for John being born in England. I realise the chn on the 1851 census were born Ireland but it doesn't necessarily follow that John was also. You need to track the movements of the 79th Reg just in case John was still with them.
Where did you find evidence of John SWANSON born c1830 being a seaman.
Do you have John with his parents on any census or other documents?
Cando
