Also not sure how a ferry would work in the wintertime
Apparently you have never seen or taken the ferry from Plattsburgh to Burlington over Lake Champlaigne which operates 365 days a year. The ice on Lake Champlaigne is a whole lot thicker than the St. Lawrence River. They have ice choppers. This is 2017 easy going, not 1865. A little bit of ice does not stop the progress and evolution of man.
This is NOT an issue whatsoever. I am sure even Rumples is familiar with the Grand Isle Ferry and they can show the folks in Montreal how it done. I daresay that if they read your post, they would be laughing. Ice is nothing to them.