Krias Yam Suf (Splitting the Sea) was not for the purpose of rescuing us from Pharaoh and his army. It was for one purpose: "Vayaaminu b’Hashem u’b’Moshe avdo — The believed in Hashem and His servant Moshe." The result was a tremendous achievement in recognition of Hashem.
They believed in Hashem before, too... But they gained a high degree of perfection now, and that was the purpose of Krias Yam Suf: Not to save the bnei Yisrael, to show them the yad (hand of) Hashem in the world. — Show Me Your Glory (Gifts of Emunah) (#E-176)