“On that day, Hashem rescued Israel from the hand of Egypt and Israel saw the Egyptians dead on the seashore. The Israelites saw the great power that Hashem had unleashed against Egypt, and the people were in awe of God and they had faith in Hashem and in his servant Moses.” 14:30-31
And Israel saw the Egyptians dead on the seashore - because the sea expelled the Egyptians onto its shore, so that Israel would not say: ‘Just as we are coming up from this side so, too, they’re coming up from the other side, at a distance from us, and they will still pursue us.’ – Rashi
And they had faith in Hashem and Moshe – Although they’d previously believed in Hashem, from the start of Moshe’s mission, their belief was now strengthened and they recognized that Moshe spoke only in Hashem’s name. – Ibn Ezra
Although the Jews had already witnessed their salvation through the Ten Plagues and the splitting of the sea, their faith reached new heights because through the events at sea, they perceived a new dimension of Hashem’s unity. This time, the very same method employed to save the Jews was simultaneously utilized to punish their tormenters. Through the splitting of the sea, Hashem accomplished a dual purpose: allowing the Jews to pass through unharmed, and drowning the Eygptians. This was further evidence of Hashem's Oneness.