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Fortunediver:A group of us dive the Niagara at least 4X per year. I am not aware of any bottle dump in the Niagara. I have found numerous blop tops, clay pipes and medicine bottles while drifting. Several are ceramic/clay bottles. There is an area known as the penny hole that is at the entrance to the rive, it can be a very dangerous dive due to boat traffic.
I have also dove the St. Lawrence in various spots and there is a bottle dump near Cornwall. It is very close to shore, but we did it by boat. Lots of bootles, old toys, tools etc. I think this was a local dump site either before or after that part of the area was flooded due to the dam being put in. As I mentioned it is very close to shore and could be a very easy shore dive. All I know is that is somewhat near Cornwall, Ontario.