Final answer:
The St. Lawrence Seaway, completed in the 1950s, is the deep water ship route joint project by the United States and Canada, linking the Great Lakes to the Atlantic Ocean.
Step-by-step explanation:
The region's most important deep water ship route - the St. Lawrence Seaway - was completed in the 1950s as a joint project by the United States and Canada. Contrary to the options given in the question, neither the Panama Canal, Suez Canal, Gulf of Mexico, nor the Atlantic Ocean fit this description, as the Panama Canal was completed in 1914 and connects the Atlantic and Pacific Oceans, the Suez Canal is located between the Mediterranean and Red Seas, the Gulf of Mexico is a body of water, and the Atlantic Ocean is an ocean, not a ship route.
The St. Lawrence Seaway is North America's most important deepwater ship route, connecting the Great Lakes to the Atlantic Ocean by way of the St. Lawrence River. This route significantly aids in the transport of both commercial goods and passengers in the region. The St. Lawrence Seaway has played a crucial role in the economic development of the Great Lakes region and continues to be a vital pathway for international shipping and trade.