The Beaver hydroplane went down north of Bergeronnes in a remote wooded area not accessible by road.
The Canadian Forces Base in Halifax dispatched a Cormorant helicopter and sent in people by parachute. Quebec's provincial police service followed later on ATVs, by helicopter and on foot.
The names of those who were aboard the plane have not yet been released.
It's the second time an Air Saguenay seaplane has been involved in a fatal crash.
In July 2010, four of the six people aboard a De Havilland Beaver died, in part because the pilot took off despite bad weather.
The airline did not immediately respond to requests for comment. Its website said it provides charters for fishing and hunting and mining exploration.