Commentary for Exodus 14:17

ואני הנני מחזק את לב מצרים, "And I, behold I will harden the heart of Egypt, etc." G'd's attribute of Mercy informed the Israelites from a sense of compassion that they should not be worried when they saw that the sea did not close after they had crossed, but remained open for the Egyptian cavalry to descend into in hot pursuit. He explained that this was only the preamble of G'd dealing with the entire Egyptian army and wiping them out.