Were a wall unto them - Compare Nahum 3:8. The waters served the purpose of an intrenchment and wall; the people could not be attacked on either flank during the transit; to the north was the water covering the whole district; to the south was the Red Sea.
Related Commentaries of Exodus 14:22
Exodus 14:22
22 And the children of Israel went into the midst of the sea upon the dry ground: and the waters were a wall unto them on their right hand, and on their left.