Thou didst divide the sea by thy strength - Margin, as in Hebrew, “break.” That is, he had by his power “broken up” the strength of the sea so that it offered no resistance to their passing through it. The allusion is evidently to the passage through the Red Sea, Exodus 14:21.
Thou brakest the heads of the dragons - Margin, “whales.” On the meaning of the word used here - תנין tannı̂yn - see the notes at Isaiah 13:22; notes at Job 30:29. It refers here, undoubtedly, to crocodiles or sea monsters. The language here is used to denote the absolute power of God as manifested over the sea when the people of Israel passed through it. It was as if by slaying all the mighty monsters of the deep that would have resisted their passage, he had made their transit entirely safe.
In the waters - That reside in the waters of the sea.