What does the Torah mean when it writes that Aharon raised his staff (a seemingly superfluous statement)?
Ramban: It means that Aharon first waved the staff in all directions 1 before striking the water of the Nile.
Thereby ensuring that all the water would be stricken and not just the water of the Nile.
What is the significance of the fact that Aharon struck the water before the eyes of Pharaoh?
Rashbam: The fact that Aharon acted as Moshe's Shali'ach, raising his staff and striking it each time upon his command, boosted Moshe's esteem in the eyes of Pharaoh.