What is the origin of the phrase "Eastern Seaboard"?

The OED has the following general meaning of seaboard:

The line where land and sea meet, the coastline; the sea-shore or the land near the sea, esp. considered with reference to its extent or configuration.

The first citation of this seaboard is from 1788:

"The Gnats are almost as troublesome here, as the moschetoes in the low-lands of the sea-board."

The OED doesn't seem to draw this connection, but I imagine this meaning came about as an extension of another meaning for seaboard, which is:

With prepositions a, at, on, to seaboard, on or to the seaward side (of a ship, etc.). Obs.

If you connect that meaning with other sea terms like overboard, it would make sense that the seaward side of a ship would be called a seaboard. And then saying that the coastline is essentially the seaboard of a landmass is a small jump.

Nowadays, it seems like "Eastern Seaboard" has become an idiom or set name for a certain region, and we don't really use it in the general sense at all (although I don't know anyone in the fishing industry).

I think back to Physical Geomorphology -- the Eastern Seaboard in the US is a shoreline of emergence and the Western is not a "seaboard" -- because the west coast is a shoreline of nothing (it is totally different geologically).

There has to be something -- some journal -- that says Eastern Seaboard. I think it has something to do with the Louisiana Purchase (someone else on the comments said it) because we did not own a west coast at the time.

I have a PhD in Geography but I cannot remember the text.