What does Isaiah 19:5 mean?

The point of Isaiah's oracles was to show the people of Judah, God's people, why they should not put their hope for safety and salvation in other nations. The Lord could bring any nation to its knees at any moment. What He wanted was for His people to trust in Him alone for salvation. He alone could give them the security they so desperately wanted.

The kings of Judah and Israel were tempted to make alliances with the kings of other nations, including Egypt. They hoped joining with these nations would give them a chance against the Assyrian—and later, Babylonian—war machines. Isaiah has already shown that Egypt will be conquered (Isaiah 19:1–4), making an alliance with them pointless. Now he shows that the Lord will also turn nature itself against the Egyptians. God will dry up the critical water resources of Egypt.

The Hebrew word translated "sea" here is yām, which can more generally mean a seashore, or the direction of the sea, or a body of water. Since the Nile river is unusually wide and slow-flowing, and its annual flooding created large wet areas, it might well have been referred to as a "sea" in that sense. That God will dry both the "sea" and the "river" suggests that the fertile Nile delta would become desolate.

In our modern era, due to a dam, the Nile River no longer floods every year. During its natural flood stage, the Nile would become a temporary "sea" in lower Egypt. This annual flooding, followed by the receding of the waters, made for some of the most fertile farmland in the world. The great famine during the days of Joseph (Genesis 41:1–4; 28–31) was likely caused by a prolonged drought that stopped this cycle. In short, the Nile River made life in Egypt possible and even prosperous. Without it, Egypt would become a desert.