The final of the Great Lakes along the St. Lawrence Seaway. Principal ports in Ontario are Toronto, Hamilton and Kingston, as well as the New York state ports of Rochester and Oswego.
Further to SgtCoolGuy's w/u below, I saw news footage of the police retreiving a stolen camaro from the waters just offshore in Mississauga. Police divers had found it while looking for something else. The car was so totally encrusted with zebra mussels that they had to scrape a huge patch off just to figure out what the make of the car was.