What was the significance of the third miracle, in connection with water, that Moshe was to perform in front of Yisrael?
Rashi: It was a sign that when Hashem would punish the Egyptians, He would begin by striking their god. 1
Seforno: Because to create a mixture from a single ingredient is totally inconceivable.
Da'as Zekenim: It was a hint that Hashem will spill the blood of Mitzrim - in [Makas] Bechoros, and at the sea.
Rashi Yashan: I.e. the River Nile which they worshipped, because it was responsible for their sustenance. And whenever Hashem punishes a nation, He always strikes their god first.
Why did Hashem repeat the word "v'Hayu"?
Ramban and Rashbam: It is the way of the Torah to repeat words in this way, 3 either for emphasis, or because of the long break between the two instances of the word.
Rashi: Otherwise, we would have thought that it turned into blood while still in Moshe's hands.
See Ramban's objection to this explanation.
Ramban: See for example Vayikra 27:3, and Devarim 18:6 (Rashbam cites Tehilim 33:3, 94:3).
Why were Yisrael more likely to believe the third sign than the first two?
Moshav Zekenim (to 4:8): The first two signs [the staff and hand] reverted back immediately. The blood did not return to water immediately.
Keli Yakar: Because it entailed striking their god, 1 which was an ominous sign of things to come - a sort of prelude of the first plague. 2
Maharal: As for what message this third sign conveyed, over and above that of the prior two, refer to 4:3:3.2.
And was more in keeping with the promise Hashem made to Avraham, "v'Gam Es ha'Goy Asher Ya'avodu, Dan Anochi" (Bereishis 15:14). See Keli Yakar, who elaborates.
QUESTIONS ON RASHI
Rashi writes: "... This teaches that it would not become blood until it was on the ground." Why is this significant? Wouldn't it be a miracle, wherever it happened to be when it changed over?
Gur Aryeh: It was imperative that Bnei Yisrael understand Moshe's words correctly. Were they to expect the water to turn to blood in Moshe's hand, and then see that it changed only when it hit the ground, they would question the veracity of the sign.
Rashi writes: "The word 'v'Hayu' (it will become) is written twice... If only the first, I might infer that the water would turn to blood while yet in his hand, and remain that way (when poured); the second now teaches us that it would not become blood until it was on the ground." Mizrachi asks - If so, the second word "v'Hayu" suffices! Why is the first one written?
Moshav Zekenim: (Do not say that the Torah could have written only 'you will pour it to the ground, and it will become blood.') If so, I would think that all the river will turn to blood!
Mizrachi: This is normal syntax in the Torah.
Gur Aryeh #1 (explaining Rashi): Were it to say only the second "v'Hayu," it would imply that the water would turn to blood only after lying on the ground for a while (for any change is expected to take time). The double "v'Hayu" indicates that it would turn to blood immediately on contact.
