The lakeshore is a coast. For border control purposes, it's a large body of water that borders the US and other countries. (Granted, there's a fairly tight bottleneck coming from Lake Huron.) There's no argument for not treating it as an oceanic border.
100 miles from the seashore also puts you into some clearly inland areas. Most locations 100 miles from the sea aren't organized around their "proximity" to the ocean.
I mean - the argument for not treating the whole thing as an oceanic border is the fact that the territorial line of US and Canada Lakes Superior, Huron, Erie and Ontario - but not at all Lake Michigan.
Lake Michigan does not touch Canada. There are shipping ports along the shores of Lake Michigan, but it's an argument that it should not be treated as an oceanic border. No matter how you measure it, Chicago is hundreds of miles from the Canadian border.
Astute readers of maps will recognize that The Upper Peninsula of Michigan gets in the way of Canada and Lake Michigan. QED: It's a fine argument among people who have looked at the map.
Those would have to be some extremely dim readers of maps. If you can read a map, and you know how water transport works, you'll see that what's getting in the way is, as I mentioned above, Lake Huron.
100 miles from the seashore also puts you into some clearly inland areas. Most locations 100 miles from the sea aren't organized around their "proximity" to the ocean.