For the horse of Pharaoh came with his chariots and with his horsemen into the sea,and Hashem brought back the waters of the sea upon them after Bnai Yisrael had crossed on dry land in the midst of the sea.
Behold, this stanza is in praise of a miracle within a miracle! Though Pharoah pursued immediatly and close behind, the waters closed in on him. Therefore at the same time Bnai Yisrael were crossing over on dry land, the Egyptians were drowning! In essence, seeing the Egyptian army drown while they themselves remained on dry land is why Bnai Yisrael started to sing. This is the concluding verse of "Az Yashir" and was also sung while Bnai Yisrael were crossing the Red Sea. (Ramban)
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