Or HaChaim on Exodus 15:10/אור החיים על שמות ט״ו:י׳

1 נשפת ברוחך, "You blew with Your wind, etc." This verse enumerates three disasters corresponding to three of Pharaoh's intentions. The reason that Pharaoh (and the Egyptians) were covered by the sea was because he had said: "I will divide the spoils." Even though under normal conditions the horses could have saved themselves by swimming, he was made to lose his own most treasured property, his horses. He and the Egyptians died a slow death as retribution for their declared intention to oppress the Jews even more harshly and to kill their leaders. This is why the Israelites referred to צללו. The Mechilta describes the words כעופרת like lead, as describing how the individual Egyptians were tossed up and down repeatedly by the waves. In their song the Israelites spoke about במים אדירים, powerful waters, to describe two features. A) the descent, i.e. צללו כעופרת; B) their inability to die at once; this is alluded to in the words במים אדירים, the powerful waves which kept tossing the Egyptians up into the air. Israel praised the Lord in recognition of all these phenomena.