Thou didst divide the sea by thy strength: thou brakest the heads of the dragons in the waters. Thou ... thou. The seven-fold repetition of the emphatic "thou" stand in majestic contrast to the powerless 'I.' Human feebleness sinks into the arms of Divine Omnipotence, and feels safe.
Thou didst divide the sea ... thou breakest the heads of the dragons in the waters. "The sea" is the Red Sea (Psalms 66:6). The "dragons" (Hebrew, taniyniym (H8577)), or crocodiles, poetically represent the ruling powers of Egypt, Pharaoh and his princes (Ezekiel 29:3-5), even as the crocodile is monarch in the waters of the Nile. Compare also, in the same connection, Isaiah 51:9-10; also Job 26:12-13.