And the serpent poured WATER like a river out of his mouth after the WOMAN,
This could have two meanings. Literally, there would be a river diverted, perhaps the Euphrates where the four ugly angels were chained (Revelation 9:14), to flood Jerusalem. Recall that there has been much geologic activity in the previous chapters of Revelation which could alter the terrain around Israel.
However, the symbolic meaning is the most likely due to the use of 'serpent' for the devil. It shows a great outpouring of demonic activity against Israel. Since demons usually reside in people (~75% water), this would imply that a flood of people will try to "drown"/kill the Jews in Israel.
That water is symbolic of evil spirits is given throughout scripture. Egypt, a type of evil master, pursued Israel to the Red Sea. God parts the WATERS and Israel, God's chosen people, crosses on DRY LAND (Exodus 14:15-20). After the wilderness wanderings when Israel crosses the Jordon River into the Promised Land, Israel crosses on DRY GROUND (Joshua 3:14-17) after the priests holding the Ark of the Covenant step into the river (full and flowing swiftly). Jesus makes a curious observation in Matthew 12:38-43-45 about demons needing WATER to survive.
So most likely, the Israeli nation will be pursued by a great demonic onslaught; the Holocaust will appear minor (birthpang - Mark 13:1-8) compared to this outpouring!
so that he might cause her to be SWEPT AWAY with the FLOOD.
We must realize that the devil was the reason for the great flood in Noah's day (Genesis 6:11-12). His reasoning must go something like this: if God can destroy all mankind (Nahum 1:8) except Noah and his family (Genesis 7:1-5) using water, I can use the same technique to destroy Israel. The devil, as believers know, is not very "creative", but tends to copy what God does (part of the deception process). The Old Testament uses this expression for destruction by an enemy (Psalm 32:6; Psalm 69:1-2; and Psalm 124:2-5). But God in His supernatural way will not let this evil onslaught approach or even touch His people (next verse).