Then the waters had overwhelmed us - Our destruction would have been as if the waves of the ocean had overwhelmed us.
The stream had gone over our soul - The torrent would have swept us away. Compare Psalm 18:4, Psalm 18:16.
Matthew Henry
Concise Bible Commentary
God suffers the enemies of his people sometimes to prevail very far against them, that his power may be seen the more in their deliverance. Happy the people whose God is Jehovah, a God all-sufficient. Besides applying this to any particular deliverance wrought in our days and the ancient times, we should have in our thoughts the great work of redemption by Jesus Christ, by which believers were rescued from Satan.
SDA Bible Commentary (limited)
Volume 1-7
5. Proud waters. As the swollen flood—waters sweep everything before them, and seem disdainful of opposing objects, so the wicked would engulf the righteous and bring about their destruction if God did not restrain them (see GC 614).