I can't sleep but I have learned a weird fact so that's something.

Both "sea" and "ocean" are words of ultimately unknown origin.

They didn't have their modern distinctions, "big body of water" and "biggest bodies of water," until later: sea is used in Old English for "the expanse of salt water that covers much of the world" as well as individual large bodies of water; it also was used of inland seas, bogs, lakes, rivers, and the Bristol Channel.

People just needed a word for "lots of water."