Micah 7:19 is the best source for God casting sins into the sea. The Scripture declares, "He will turn again, he will have compassion upon us; he will subdue our iniquities; and thou wilt cast all their sin into the depths of the sea."
The Bible lists several verses that are similar in text. However, there is no one verse that states the phrase exactly. There is no physical sea of forgetfulness; rather, the sea mentioned in Micah is interpreted as the concept of God forgiving all sins. He actually erases the sins from his memory, never to bring them to mind again. In this case, God's mind is the "Sea of Forgetfulness."