That sendeth ambassadors
The local reference is evidently to an embassy from Egypt, resulting in the alliance denounced in Isaiah 30, 31, (Jeremiah 37:7-11).
Related Commentaries of Isaiah 18:2
Isaiah 18:2
2 That sendeth ambassadors by the sea, even in vessels of bulrushes upon the waters, saying, Go, ye swift messengers, to a nation scattereda and peeled, to a people terrible from their beginning hitherto; a nation meted out and trodden down, whose land the rivers have spoiled!