Why would God perform miracles that the Egyptian magicians could seemingly duplicate (7:11, 22), like turning staffs into snakes and water into blood? And why would God bother to perform miracles if He knew that they would not convince Pharaoh (7:4?)
About turning the Nile River into blood, keep in mind that the Egyptians worshiped the Nile as a god. By turning the Nile into blood, God was communicating that He had power over the Nile, that He alone was the one true God, and that life ultimately comes from Him, not the Nile.
Also, compared to some of the later plagues, turning staffs into snakes and water into blood were relatively gentle signs (and thus perhaps easier for the Egyptian magicians to try to duplicate using their secret arts). I believe God started with the gentler miracles because God is gentle and wants to give as many people as possible a chance to believe in Him before it was too late.
God already knew that Pharaoh would not turn to Him based on the first few miracles, but God also knew that others - both Egyptians and Israelites alike - were watching and could turn to faith in Him. So rather than immediately resort to a miracle so severe that it would kill those who might not yet have believed, God chose to go with something gentler, such as turning all surface water into blood but not touching the underground water.
By starting with gentler miracles, God was knocking on the door of the people's hearts, instead of barging in by force.
God is patient and gentle with us. He gives us time to get on His side. He gives us second chances. When you experience a miracle from God, don't harden your heart like Pharaoh or rely on people's rationalizations about what happened. Instead thank God and give your heart to Him. The sooner you do so, the better off you and those in your care will be.