Funny, because the Celts, who came up with the idea of thin spaces long before it was cool, did see coastlines as a kind of border between worlds (in a supernatural way, not just the mundane border between land and sea).
Yeah as I was making my shitpost I realized that a coastline entirely meets the actual definition of "liminal" (as opposed to the common usage, "teens pretending to be scared by an empty office break room")