The only dredging being done on the Great Lakes is to existing waterways and is not being done, nor is capable of being done, to accomodate larger ocean craft. The lakes have been low for some time and the extremely dry winter last coupled with a very dry spring and summer have caused the water table to drop even further. I've observed a number of older wells where the water is as low as it has ever been within them. The good news is that the water table rebounds and the entire process is cyclical.
The recharge to aquifers, and the lakes by extension, in the Great Lakes region dwarfs water withdrawals. We're not running out of water and it wasn't that long ago (1970-80's) that all the cottages were falling into Lake Michigan because of the high water levels.
For what it's worth, the Welland Canal is the limiting factor on ocean going vessels entering the Great Lakes. With a controlling depth of 28 feet fresh water, which produces an average salt water draft of 27'-02", the Great Lakes are not a viable economic port of entry for deep sea trade. The Mississippi to New Orleans is much more viable via tug and barge.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.