Isaiah 33:21 - Ellicott's Commentary On The Whole Bible

A place of broad rivers and streams... — Better, rivers and canals. The bold imagery has its starting-point in what the prophet had heard of the great cities of the Tigris and Euphrates. What those rivers were to Nineveh and Babylon, that the presence of Jehovah would be to Jerusalem, that could boast only of the softly going waters of Shiloah (Isaiah 8:6). Here, again, we have an echo of Psalms 46 : “There is a river, the streams whereof shall make glad the city of God.” The words help us to understand the symbolism of Ezekiel’s vision of the “river that could not be passed over,” flowing out of the Temple (Ezekiel 47:1-5). And the spiritual river of the Divine Presence would have this advantage over those of which the great cities boasted, that no hostile fleet, no pirate ships, could use it for their attacks. So in Psalms 48:7 the “ships of Tarshish” are probably to be taken ‘figuratively rather than literally’ for the Assyrian forces.