Hi John.
Cannot be of great help as the definition of waterman,, boatman,, flat man can get totally confusing.
from my experience and thats not a lot.... I have Canal Boatmen in my family.They lived and worked on canal barges and they used the canal system as their highway.
In the Northwich area we have the River Weaver. and the men that used that river had boats that were called River flats......design I expect ... the men that used those boats on the River Weaver were called Weaver flat-men. they didn't usually go on the canal and the canal men didn't usually go on the river...and until the Anderton Boat lift there was no connection between the two unless you went to the sea. [ think I am right in saying this?][ even after the connection the men didn;t redefine their names...canal was canal and flat was flat ]
The Manchester Ship canal was built in the 1800's to transport ships from the Mersey River to the centre of Manchester. it is like a wide Canal that first of all runs alongside the Mersey and then makes it own way to the centre of Manchester.
The ships that used it were much bigger than canal barges.[ In fact in later years huge Ocean going ships used it ... but thats another story]
surfice to say that your Thomas might well have used the Manchester Ship canal and that when asked what his occupation was on Census night he could have said that he was a Waterman [ because he worked the water system ] and that he was in charge of a Barge.....
I don't think there is any underlying secrets about what he was or did..................he was a man that worked on the waterways of Cheshire/Lancashire and he was termed as a waterman or a flatman ...what he doesn't appear to be is a Canal Boatman.
Maybe this is some help
?
Sallie