Never sailed up the St. Lawrence but live on the river in the Thousand Islands. Have traveled extensively throughout this area from Gaspe, Qc. and throughout the Great Lakes. From what I understand, the worst currents are from Quebec
city to the bottom of Lake Ontario
and I know that it can run 7kts at Ivy Lea going under the International Bridge there. Have seen many underpowered sailboats fighting the current
for hours and making little headway.
Big Tides east of Quebec
City so an incoming tide will slow the current down and progress can be good up till there. After that you will be fighting upstream for 300 miles or so. Beautiful scenery that you won't see going the US side and having seen a lot of the world there are not many places that compare.
Winds are generally out of the NW to W but the river runs SW most of the way so you should be able to do quite a bit of sailing or motor
sailing a least. Good Luck you should have a great time and you will especially love the people in the Maritimes.