The 295-foot-long submarine was being towed from the Heddle Marine Shipyard in Hamilton through the canal to Port Burwell.
Onlookers pressed up against fences at Lock 1, stood shoulder to shoulder on the Homer Bridge, crammed a viewing platform at Lock 3 and dotted trails at all points in between.
Niagara Regional Police set up cruisers in high-traffic areas due to heavy numbers of pedestrians crossing roads like Lakeshore.
Side streets were crammed with cars and the shoulder of the Welland Canal Parkway was a parking lot as sub hunters staked out their spots for a closeup view.
In May, it was towed from Halifax through the St. Lawrence Seaway to Hamilton, where it was repainted.
On Sunday, it began its journey on a barge towed by two tug boats to Lake Erie, where it will find a home at the Elgin Military Museum of Naval History.