In Kipling's "The Land" he writes:
Then did robbers enter Britain from across the Northern main
And our Lower River-field was won by Ogier the Dane.
Here "main" seems to mean sea, i.e. the North Sea.
This seems to me to allude to the Spanish Main, but from what I can find that phrase actually refers to the mainland, not the sea.
The OED (1st ed) has "4. Said of a considerable, uninterrupted strech of land or water" (link) but the examples given mostly seem to be land.
Webster's 1913 has "main, n. 3 ... the high sea; the ocean" (link), but again the examples seem mostly to be of land, including "the main of Spain" to mean mainland.
etymonline doesn't have this usage of "main"
What is the etymology of this usage? Is it a corruption from "Spanish Main" coming to mean sea instead of "mainland"?