Literary analysis: Verse 7 is the literary and symbolic contrast to verse 6. Because Judah refuses the "gentle waters" of the Lord, they will be visited by the "strong and many" waters of the Assyrians.
Isaiah's poetic typology is a powerful statement in both literature and theology. Water is an essential element of life. The "gentle waters" of Shiloah provide water for the physical salvation of thirsty Jerusalem. The "strong and many" waters of the Assyrians are also life-givers or preservers. In that the Assyrians quell the belligerent intentions of Syria and Israel they also provide a measure of salvation.
Isaiah's image is calculated to make a more important point, however. The contrast between the "gentle" and the "strong and many" is precisely one of beneficial versus destructive. Even though water is essential, in too great a quantity, it is also massively destructive. When Isaiah couches the coming of the Assyrians to water overflowing its banks, the image is one of the destruction of a flood. While water may be essential, the "gentle" water provides that critical element safely and reliably. The flood provides water, but carries with the water a tremendous devastation. So will the Assyrians, temporarily the heroes or delivering Jerusalem from Syria and Israel, will become a plague in and of themselves. They bring to Jerusalem not ultimate salvation, but imminent destruction.