Why did Hashem instruct Aharon to strike the water for the first two Makos?
Rashi: Because the water protected Moshe when his mother placed his casket in the river (See Shemos 2:4). Consequently, for him to strike it would have demonstrated a lack of gratitude.
Moshav Zekenim (8:2): Aharon was Moshe's Shali'ach, so he performed Makos from the land. Moshe himself performed what came from above.