Albert Barnes Commentary Exodus 14:22

"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." — Exodus 14:22 (ASV)

Were a wall unto them - Compare Nahum 3:8. The waters served the purpose of an entrenchment 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.