From Plato's Critias:
“And he (Poseidon) named them all; the eldest, who was the first king, he named Atlas, and after him the whole island and the ocean were called Atlantic. ”
Excerpt From: Plato. “Critias.” iBooks. https://itunes.apple.com/us/book/critias/id498662802?mt=11
Here, Plato explains the origin of the name Atlantis and the origin of the name for the Atlantic Ocean.