Isaiah continually brings up the memory of how God saved the Israelites from their Egyptian persecutors during the Exodus—parting the Red Sea (or "Sea of Reeds," if you want to get all technical) and allowing them to pass through, before drowning the Egyptian army:
Where is the one who brought them up out of the sea with the shepherds of his flock? Where is the one who put within them his holy spirit, who caused his glorious arm to march at the right hand of Moses, who divided the waters before them to make for himself an everlasting name, who led them through the depths? (Isaiah 63:11-13).
Crossing through the sea becomes more than just an event from Israel's past. It's a metaphor for any passage through chaos and destruction towards peace, wholeness, and holiness. The American poet Robert Hayden used the same sea-crossing metaphor in his poem "Middle Passage" to describe the journey of enslaved Africans on the slave ships, through the experience of slavery itself and into a future of freedom and peace: "Middle Passage: / voyage through death / to life upon these shores."
Suffering from the Assyrian invasion and a future exile in Babylon, Isaiah and Israel look back to their origins, considering the God who saved them from chaos and slavery before by parting the sea. It becomes an image of their liberation, of any people's liberation really. A famous study of the American Civil Rights movement is entitled Parting the Waters: America in the King Years 1954-63.