The Census is the obvious place to start, though very few boatmen appear, certainly far less than worked on the canal. The enumerator would not have visited local canal wharves at the appointed time, so when asking people on boats if they were there at a particular time, the answer would usually have been 'No', and hence the lack of boatmen recorded in the Census.
<Read more at the Leeds and Liverpool Canal Society' s website - http://www.llcs.org.uk/FamilyHistories.htm