A profound promise of absolute security for Jerusalem marks the exact midpoint of the Book of Isaiah [מנחת שי]. The assurance is that God will reveal Himself as a mighty and powerful force, actively standing ready to help the city [רד"ק, מצודת דוד, מצודת ציון, שד"ל, ביאור שטיינזלץ]. This guarantee either directly ensures that only good, and no harm, will reach the people [רש"י], or it serves as a condition, meaning the city will be saved only if God is embraced as its ultimate fortress [מלבי"ם].
To illustrate this divine protection, Jerusalem is pictured as being surrounded on all sides by wide rivers and man-made canals [אבן עזרא, מצודת ציון]. The primary approach among commentators understands this imagery as a powerful metaphor. God's presence will shield the city exactly as if it were circled by massive, uncrossable bodies of water that block any enemy advance [רד"ק, מצודת דוד, אבן עזרא, ביאור שטיינזלץ]. Conversely, another perspective reads this as a literal vision of the future. In this view, actual rivers will one day flow from Jerusalem, but unlike ordinary waterways that might allow enemy fleets to navigate and invade, these waters will form an entirely impassable barrier [רש"י, אברבנאל].
This barrier will perfectly repel any naval threat, whether from small, light rowboats [רד"ק, אבן עזרא, מלבי"ם]—such as nimble pirate vessels hunting for loot [שד"ל]—or from large, heavy, and powerful warships [רש"י, מצודת ציון, מלבי"ם, אברבנאל]. The natural danger of such vast rivers explains this defense: broad waters are often swept by severe storms that can easily capsize light boats, while massive warships require deep channels and run the risk of sinking into mud and sand without an expert navigator [שד"ל].
On a historical and symbolic level, the threatening waters and ships represent the Assyrian army. The King of Assyria viewed himself as an overwhelming river or a mighty fleet ready to drown the small city of Jerusalem. However, he will ultimately discover that God provides a far greater and stronger defense, one that no Assyrian military force can ever breach [רד"ק, מלבי"ם]. Because of this divine shield, Jerusalem will experience complete peace, entirely free from the fear of any enemy approaching from land or sea [אבן עזרא].