Or HaChaim on Exodus 4:9/אור החיים על שמות ד׳:ט׳

1 והיה אם לא יאמינו גם לשני האותות, "In the event that they will not believe even both these two signs, etc.," This sounds strange. Does G'd then entertain any doubts so that He has to phrase this verse in such a way? Besides, G'd had already told Moses that the people would listen to his voice! Perhaps the thrust of the message is addressed to Moses' concerns, and G'd is saying to him: "In the event you are afraid that even these two miracles will not be sufficient to convince the Israelites of your authenticity and the authenticity of the message of the imminent redemption, I will let you perform another miracle which will be the clincher and as a result of which they will certainly believe you."

2G'd may also have wanted to tell Moses not to worry about aspects of the people's faith which could not be discerned on their faces. In the event that Moses worried that though externally the people would appear to believe both in him and in the success of his mission, they might harbour reservations which Moses could not detect. G'd reassured him on that score and told him that as a result of all three miracles they would believe him without reservations.

3והיו המים….לדם ביבשת. "The waters…will turn into blood on the ground." Rashi has enlightened us by remarking that the repeated use of the word והיו in this verse indicates that the waters will not turn into blood until they touch the dry land. I only wish to add one more comment concerning the statement of our sages in Shemot Rabbah 9,10 that G'd eventually commanded Aaron and not Moses to perform this miracle in front of Pharaoh because the Nile had saved Moses' life at the time his mother placed him amongst the reeds and it would have been gross ingratitude to strike at the part of nature which had saved his life. In this case where G'd commanded Moses to strike the waters personally, He did so with the proviso that the waters would become blood only after they had already ceased to perform their life-giving and life-preserving function, i.e. after they had been spilled on the ground.