Hi
From the baptism of on of his son's it does say he was a mariner.
Given the death date (1873 aged 63) that bevj has found, there is a masters/mate certificate issued in Sunderland for a George Duncan that does seem to fit.
The Masters Cert. of service states 'George Duncan born at Sandwick Orkney 1810, in the coastal and foreign trade for 41 years, issue date 1871'
claire