Manson's: Leith 1871

[bmanson]

I have lost my widowed ggg grandfather John Manson.  He appears on the 1861 census, age  58, living at No. 11 Sheriff Brae, South Leith. We believe he was still there when his daughter-in-law and his 2nd daughter died in 1866, but he was "deceased" by the 1874 marriage of another daughter. Scottish researchers are certain he died "outwith" Scotland during that period.
However, his son Robert (b. 1835 Leith) and grandsons John (b. 1859 Glasgow) and  James (b. 1862 Leith) we know were in at least Edinburgh or Leith during this period. Family myth has it that after Robert's wife had died in 1866, Robert and John went to sea, while James was left with a foster family.  In any event, we lose the whole lot of them until the 1881 census where Robert shows up back on Sheriffs Brae in Leith and James shows up boarding in Jamestown (no sign of son John has ever been found).
Is there any way of checking mariners "at-sea" during the 1871 census?  any suggestions as where I go from here.