All in all, I'm inclined to think that someone was pulling names and occupations out of a hat for James' dc...
Just to cover all possibilities, I tried searching for boatmen named McNab in the census.
There is a Robert McNabb, boatman, in Denny, Stirlingshire in 1841, age 45, born Stirlingshire
His family is Margaret McNabb, 45, born Stirlingshire
Alexr McNabb, 15, born Stirlingshire
Helen McNabb, 12, born Stirlingshire
Mary McNabb, 8, born Stirlingshire
Thomas McNabb, 1, born Stirlingshire
In 1851 he is in Kilsyth, Stirling, one of TWO Robert McNab, boatmen...
Robert McNab, 50, boatman, born in Falkirk, Stirling
Margt McNab, 50, born Denny
Thomas McNab, 10, born Kilsyth
Agnes McNab, 7. born Kilsyth
John McNab, 4, born Kilsyth
Robert McNab #2 in Kilsyth, Stirling, in 1851, 58, boatman, born Pre??, Stirling
Mary McNab, 50, born Denny
Agnes McNab, 15, born Kilsyth
Thomas McNab, 10, born Kilsyth
William McNab, 7, born Kilsyth
Then there is Archibald Beaton, in Tobermory, Argyll in 1841, 50, born in Argyll, who is also a boatman
His family consists of:
Cathrine Beaton, 45, born in Argyll
Jessie Beaton, 11, born in Argyll
Cathrine Beaton, 7, born in Argyll
There is no rod* who is a boatman... Could William have had a speech impediment that made "Robert" come out sounding like "Roderick" and "Mary" or "Margaret" come out like "Euphemia"?

Sorry, I'm not being very helpful
And then there is the Roderick Beaton found by Annette, indexed as "Beason" in the ancestry database in 1841, although the rest of his family are all "Beaton"
Rodrick Beason, 45, turner, in Barony, born in Scotland
Jane Beaton, 40, born in Scotland
Daniel Beaton, 15, born in Lanarkshire
Sarah Beaton, 15, born in Lanarkshire
John Beaton, 10, born in Lanarkshire
Rodrick Beaton, 5, born in Lanarkshire
None of these folk seem to have anything to do with your James. I can understand why your head's spinning! Unless Roddy and Effie came from Ireland? In which case, good luck!
In short, it seems too much of a coincidence that two of James' daughters were Elizabeth Reid and Margaret Shearer. He's got to belong to that family. No idea where that info on his dc came from though

k
Anybody else have a bright idea?
Sarah