I took a tug and a couple of barges from Muskegon, MI to Norfolk, VA via the seaway back in the early eighties-a vacation job. If memory serves me right it took about a week to ten days at 5-6 knots.
It's a beautiful trip right on through the strait at Canso. After that you might put in at Halifax but once you pass Cape Sable you're fully at the tender mercies of the North Atlantic until Cape Cod. That leg is long enough to catch some weather regardless of how well planned the trip. They'll run into some areas of the St Lawrence River, mostly between the eastern end of Lake Ontario and Montreal, where heavy fog occurs. It's sometimes best to just find a convenient wharf and tie up for a few hours. As I recall, the fog is less prevalent after Montreal.
And, if you know them, tell them to be sure not to miss the bridge near Montreal (I can't remember if it's east or west of Montreal) that you go under and over at the same time!
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.