New York City and Montreal Share a Waterway to the Atlantic

New York City (USA) and Montreal (Canada) are connected to the Atlantic Ocean via the St. Lawrence River / Great Lakes system.

The Water Route

1. Montreal sits on the St. Lawrence River at the head of deep-draft navigation.
2. The St. Lawrence flows northeast → Lake Ontario → Welland Canal (bypassing Niagara Falls) → Lake Erie → Detroit River → Lake St. Clair → St. Clair River → Lake Huron → St. Marys River / Soo Locks → Lake Superior (western terminus).
3. Eastbound from Montreal, the St. Lawrence widens into the Gulf of St. Lawrence → Atlantic Ocean.
4. New York City sits at the mouth of the Hudson River, which flows into New York Harbor → Atlantic Ocean.

The "Common Waterway" Link

While NYC and Montreal are on different rivers (Hudson vs. St. Lawrence), they share access to the Atlantic Ocean and are linked by the Great Lakes–St. Lawrence Seaway (opened 1959). A ship can sail from the Great Lakes to Montreal, out the St. Lawrence to the Atlantic, and down the coast to New York Harbor—all on connected, navigable waters.

Thus, they are the two major North American cities that "share a common waterway to the Atlantic" in the sense of a continuous inland–ocean shipping network.