What is the significance of the statement "With this you will know that I am Hashem"?
Seforno: Par'oh would now learn that Hashem (via the Name Havayah) manipulates the laws of nature, to destroy the Nile which is naturally indestructible. 1
Rashbam: It is the answer to Par'oh's statement (in Sh'mos, 5:2) "I do not know Hashem" - Now he was about to receive practical demonstration as to who Hashem was.
Riva: Par'oh [made himself a god, and] said "the river is mine, and I made it." 'Now you will see that I am G-d, and not you, for I will strike the river, and you will be unable to save it'.
Why did Hashem begin the plagues with the turning of the water into blood?
Rashi: Because, due to the fact that there is no rain in Egypt and they rely totally on the Nile overflowing its banks for their water supply, the Egyptians worship the Nile. So Hashem struck their god before striking them.
Oznayim la'Torah #1: It was a punishment for the blood of the Jewish babies who were thrown into the River. 1
Oznayim la'Torah #2: Refer to 7:21:1:1.
It seems to me that the punishment for drowning the Jewish babies was the drowning of the Egyptians at the Yam-Suf
Why did Hashem tell Moshe to say "with the staff in my hand"?