Why did Hashem instruct Aharon to strike the water for the first two Makos?
Rashi and Targum Yonasan: Because the water protected Moshe when his mother placed his basket in the river (See Shemos 2:4). Consequently, for him to strike it would have demonstrated a lack of gratitude. 1
Moshav Zekenim (to 8:2): Aharon was Moshe's Shali'ach, so he performed the Makos that struck the land. Moshe himself performed the Makos that came from the sky.
Gur Aryeh: (Why does that make a difference? The water is but an inanimate part of Creation!) Rather, the Torah is teaching us proper conduct -- "Do not throw a clod of earth into the well that you have drank from" (Bava Kama 92b).
What are the connotations of "u'va'Eitzim u'va'Avanim"?
Rashi and Targum Yonasan: It means the water that had been poured into wooden and stone vessels, would turn to blood as well.
Oznayim la'Torah (citing Midrash ha'Gadol): It means the blood would actually ooze out of wood and stone. 1
Oznayim la'Torah: This was measure-for-measure, because the Egyptians forced the Jewish slaves to place babies in the walls when they were short of bricks.
