I would look into Dolphin Insurance. They are located in Vancouver although their website indicates they provide coverage on the Great Lakes. (That may mean though that they only provide coverage for local boats that travel to the Great Lakes.)
In any event we've used them for a few years, had one claim (towing) that they handled very quickly and helpfully with no fuss, no bother. They are great to deal with.
Dolphin Insurance Services Inc. - Specialty Marine Insurance