This entire thing is silly - they've been armed since 2004 when the Rush-Bagot Treaty interpretation was amended by
both Canada and the United States to allow
both countries to arm their coast guard vessels on the Great Lakes with a weapon up to .50-calibre in size. The ban on military vessels remains in effect. It's a smart and prudent move, one which Canada should match given the uselessness of our Coast Guard (as it isn't a paramilitary force like the RCMP) - not to mention the border guard.
Armed coast guard vessels on the Great Lakes? Those Americans are so warmongering and pigheaded, what will they do next? Have armed fighter jets patrol their skies? It's not like terrorists are ever going to fly planes into buildings.
Oh, and the St. Lawrence Seaway is the legal property of both Canada and the United States. We initially were going to pay for it ourselves and have sole ownership, but the Americans ponied up their share of the cash so that Canada wouldn't be able to claim the seaway as an internal waterway - and Canada was more than happy to accept that offer as it saved us a lot of money better spent elsewhere.
Canada has no jurisdiction, legally or internationally, to bar the United States from using the SL Seaway and vice versa. Contrary to the arguments of DerbyX, Canada nor the US could ever bar the other from using the seaway - the country that did so wouldn't have a leg to stand on internationally. It really is an international seaway, though exclusive to two parties instead of the entire world.