"Then say to him, 'The Lord, the God of the Hebrews, has sent me to say to you: Let my people go, so that they may worship me in the wilderness. But until now you have not listened. This is what the Lord says: By this you will know that I am the Lord: With the staff that is in my hand I will strike the water of the Nile, and it will be changed into blood. The fish in the Nile will die, and the river will stink; the Egyptians will not be able to drink its water.'" NIV translation
The warning that Pharaoh was given would have been a direct statement that God was greater than Hapi who was their god of the Nile. They had declared that Hapi was the creator of all things but Moses warned Pharaoh that he was going to see the power of the true Creator.
"The Lord said to Moses, 'Tell Aaron, "Take your staff and stretch out your hand over the waters of Egypt - over the streams and canals, over the ponds and all the reservoirs - and they will turn to blood." Blood will be everywhere in Egypt, even in vessels of wood and stone.'" NIV translation
We see that not only the Nile was going to be affected but all water everywhere. Throughout the Bible, blood represents life and this was to show that God is the giver of life. This was to show that God had control over all things and not just the river like their "Nile god".
"Moses and Aaron did just as the Lord had commanded. He raised his staff in the presence of Pharaoh and his officials and struck the water of the Nile, and all the water was changed into blood. The fish in the Nile died, and the river smelled so bad that the Egyptians could not drink its water. Blood was everywhere in Egypt. But the Egyptian magicians did the same things by their secret arts, and Pharaoh's heart became hard; he would not listen to Moses and Aaron, just as the Lord had said. Instead, he turned and went into his palace, and did not even take this to heart. And all the Egyptians dug along the Nile to get drinking water, because they could not drink the water of the river. Seven days passed after the Lord struck the Nile." NIV translation
Events took place just as God had described and revealed what was in the heart of Pharaoh.