Albert Barnes Commentary Psalms 124:4

"Then the waters had overwhelmed us, The stream had gone over our soul;" — Psalms 124:4 (ASV)

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 Psalms 18:4 and Psalms 18:16.