My wife and I have sailed the St Lawrence in both directions between
Halifax, NS and
Lake Ontario in an
Endeavour 38 sailboat. The upstream trip is doable but you have to
work with the tides and currents. The document you need is produced by the Canadian Hydrographic
Service (CHS) called "Atlas of Tidal Currents (St. Lawrence Estuary)". This shows a pictorial representation of the currents linked to the times of high and low tide. I consider it mandatory. Your
route will be east along the south coast of
Nova Scotia, north-west between mainland
Nova Scotia and Cape Breton Island to the Gaspe. Follow the south shore of the St. Lawrence, cross to the North Shore at Tadoussac, the Saguenay River. You will have to stop at Quebec City (lock in to the marina) and catch the lock to make the run west past the worst current. Towns along the St. Lawrence River are 30 to 50 miles apart to make
day sailing reasonable. Plan ahead. Hope this helps.