The sea saw it, and fled. "The sea" is the Red Sea. It "looked," and saw God leading his people (Exodus 14:19-24), and then at once "fled," and left a dry channel as "a way for the ransomed to pass over."
Jordan (literally, the Jordan) was driven back (comp. Joshua 3:13-17). These two marvels "marked respectively the beginning and the end of Israel's long journey" (Cheyne). They were parallel facts, and are naturally alluded to together (comp.