There are cruises from Quebec city or from the east coast of the US (Boston, New York) that stop at the old port of Montreal. Holland America and Crystal Cruises offer cruises to Montreal by way of New York, Boston, Halifax, etc (mostly in autumn for the fall foliage).
If you own a yatch or sailboat, there are marinas where you can dock either in the old port or at Ile Ste-Hélène .
There are also some shipping companies that let you hop on their freight ships (I'm talking about legitimate travel...), it's cheap but it's not the most comfortable or fastest way to travel.