Telnyk is right, and the rest of you don't seem to understand what the Erie Canal was (I have no idea why one of you called it the Mohawk-Erie... that's not a name of any canal and the Mohawk River was not used as part of the canal until the 20th century). The Erie Canal was literally a ditch I could stand in with my head above water and I could (in my youth) have jumped over easily. It never used the Mohawk River and in fact used aqueduct bridges to pass OVER the Mohawk at several locations. Donkeys pulled rafts. No single ocean going ship ever went past Albany (or more specifically Troy where the tide ends on the Hudson Estuary and sometimes Waterford during high water). This is the technology of canals in the time period we are talking about. Your canals around rapids on the St Lawrence were no different and your talking about a much longer route, with more time closed due to ice and weather. You are acting as if a clipper or even a yacht or sailboat is going down the St Lawrence. It isn't anytime before the 1900s. Period. End of discussion. It can't happen, so stop with this Canada can do it crap I constantly see regarding this canal, it's getting too much and it shows a lack of knowledge regarding canals and real history.
Actually I think you're failing to understand both the complexity of the existing canals built in Canada, and the difference between the Erie Canal and a proposed St. Lawrence canal. Principally that the canals built historically by the 1860s (Lachine, Buhearnois, Rideau) were greater in depth that the Erie Canal ever was, and these can all use the St. Lawrence River rather than having to depend purely on a canal project that has to go over rivers.
What is suggested here is something that will allow steamers and river boats to shoot the St. Lawrence with ease compared to the difficult runs of OTL and allowing ships in excess of 500 tons to transit the canals. OTL that was possible by the 1850s, so maybe by the 1860s-80s you can expand them to accommodate ships in excess of 1000 tons. Nothing about the proposed "seaway" is outrageous for the technology of the time. We're not talking about the 10,000+ ton cargo ships of today, just smaller ocean going clippers or steamers. Even then most of those will stop at Montreal/Halifax then move their cargoes to river boats.
This isn't impossible for the technology of the period.