The majority of'tourists' travelling between Toronto and the Falls are from the GTA itself, no? So yes, I think that many would incorporate a stop at a Burlington/Hamilton aquarium as part of a family vacation, or would make it a day trip on its own.
As for foreign visitors it all depends on how the aquarium is promoted and how significant a site it is, as to whether they would seek it out in significant numbers. Bus tours might add it into their itinerary or offer it as an option, and it could be promoted in tourist marketing in tandem with Toronto/Niagara Falls sites. Besides, in this day of google it could be easily arranged that a cursery search of Toronto would yield a Burlington/Hamilton aquarium result. Unlike many other tourist-friendly sites in the GTA (Wonderland, Black Creek, etc) Burlington is easily accessible from Union Station on the Go Train line... and here in 40 minutes!
The idea for Hamilton/Burlington, however, would be to promote the area to GTAers and southwestern Ontario as a solid day-trip or weekend destination in tandem with the RBG, Dundurn Castle and a potential Robert Bateman museum.