I (my spouse and I) traveled the length of the St. Lawrence back in 2017 when we sailed all the way to
Newfoundland. Happy to share what I learned.
Your message is a little unclear, but I think you're heading down to Montreal from
Lake Ontario (not up). Not sure what info you're after. Feel free to ask away...
If you're going this season I bet you'll be dealing with higher-than-normal downstream flows. Just like in 2017,
Lake Ontario is high, so they'll likely be letting out more
water than normal. We found this increased the normal downstream currents by at least a
knot. So you can expect to feel 1-3
knot currents most of the time, and even more in some tight areas.
Other than that there are seven locks on the
route if you are going all the way to Montreal. I think they require that pleasure boats have at least two people on board to go through the locks. They're not hard, and going down is easier than going up.
What else?