I’m an open boater – so I don’t know the kayaking system too well (which is I assume what you’re interested in).
But…I know at MKC (and I think also Paddler’s Co-op) which are two of the main paddling schools in the Ottawa valley, they don’t require an official certification for their kayak instructors (although they do for their canoe instructors). Some folks are working on coming up with a new certification system, which may be required to instruct in the future, but it isn’t yet. For the time being, if you’re a kayaker who hasn’t instructed at the school before (in this case I’m just talking MKC – I don’t know about the Co-op or other schools), you just work as an assistant for one course and after that they either give you the okay to instruct on your own or not.
If you’re interested more from an insurance point of view, I really don’t know how that works. Like I said, I’m an open boater, so I pay an annual membership to ORCA (ontario recreational canoeing association) and that covers me while I’m teaching. For kayakers I’m not sure (but anywhere you are looking to work should be able to tell you).
Hope that helps to some extent – but another kayaker would probably be able to tell you more…