Never bring back...
And understand who cast our sins in the depths of the sea? They have been cast there by God Himself, and what He has cast out by His hand, His hand will never bring back.
If you and I had taken our sins and cast them into the sea, they would have been found again. It is like a murderer who tries to hide the dead body of his victim by casting it into the sea. But after a day or two the dead body will float to testify against the murderer.
Not so with God, when He cast out our sins into the depths of the sea, they will NEVER float again. They are forever buried in the depths of the sea of forgiveness to be recovered and remembered no more.
I believe the sea here has reference to the fountain filled with blood of Christ which cleanses us from all sins, which has washed away all iniquity, purged all transgression, cast all the sins of God's heritage behind His back, and drowned them in a sea unfathomable of grace, mercy, and love.
Satan may try to bring them up again and again, but if it is washed by the blood of Jesus Christ, he cannot do so. [Greg Hocson]