Maybe for other visa offices, but for Mexico the info is quite clear:
http://www.cic.gc.ca/english/pdf/kits/forms/IMM5878E.pdf
If your stop-over (transit) in Canada is less than 48 hours, provide a copy of the itinerary and the visa to the destination country, if applicable. If you take a cruise that will stop at a Canadian port, you must apply for a Temporary Resident Visa (not a transit visa). The TRV is required, whether you intend to go ashore or not.
Copy of itinerary is enough, but booking is preferred.