Lock 3 in St. Catharines is the most accessible place to watch ships climb the Welland Canal. From the viewing platform, freighters seem almost absurdly large as they edge into the lock and rise or fall between the Great Lakes.
The St. Catharines Museum and Welland Canals Centre adds useful context, especially if you have only a vague idea of how the canal changed the region. Ship schedules vary, so check ahead if seeing a transit matters to you.