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"A" coast. This also makes a distinction between the Great Lakes and... every other lake in the US. It's not salt water, because, well, the Great Lakes. So what determined whether a lake was big enough to qualify as having "a shore"? The Great Salt Lake in Utah is the 7th largest lake in the US (3 down from Lake Ontario, which is represented) and doesn't appear to be be considered.
Are these "shores as US national borders, except for the shores of rivers?"
The great lakes are large enough to not see the other shore/coast due to the curvature of the Earth and they are on an external border which makes them comparable to inland seas despite being freshwater.
The great lakes are comparabke to the Red Sea or Black Sea.
The great salt lake doesn't connect to the ocean or any other shore and therefore is not a major shipping route. Also it may only be two more down than Ontario but it's an order of magnitude smaller.
So, the definition is "a body of water from which one can navigate - even if by river or canal - to an ocean"? How big does the traversal have to be - if I can get there by canoe without portage, does that count? What about Lake Eirie? AFAIK, you can't navigate without portage from Eirie to the Atlantic, but it connects to Lake Ontario via the Niagara River.
...international coastlines; shores with ports-of-entry...