It would seem clear that the Canadians are simply striking the definition of "vessel" that is provided in the colregs.
That probably means there is a different definition someplace in Canadian statutes. Seaplanes may be exempted, ground-effect aircraft may be handled differently...
Without knowing the purpose of what you are trying to teach to what kind of students, one can only suggest asking them "OK, why do you think Canada
is striking this definition?" and see if they figure it out. Or if they can figure out the governing authority and that asking them directly may be the most reliable answer.