A second question is how extensive was the plague? 7:17,18,20,21, refer to the Nile, while 7:19,21 record that there was blood everywhere in Egypt. Was the plague just with regard to the water in the Nile or all waters in Egypt?
A third question is that if all the water turned to blood, how could 7:22 record that the Egyptian magicians were also able to turn water into blood? How did they have any water? On the other hand if the plague was only by the Nile why does 7:24 record that everybody in Egypt had to dig for water around the Nile to get drinking water? Where there no other sources of water? We will start with the first question.
Rashi (on 7:19) quotes the Midrash (Shemot Rabbah 9:10) that Aharon was the agent of the plague since Moshe could not hit the Nile since the Nile had saved him when he was a baby. Following this approach, Ibn Ezra and Ramban (on 17:5) claim that 17:5 does not imply that Moshe hit the water, but that in 17:5 G-d was telling Moshe to take the stick that Moshe commanded Aharon to hit the Nile. This is not the simple understanding of 17:5.
Abravanel (1997, pp. 105,106) writes that both Moshe and Aharon were the agents of the first plague. Moshe’s blow was in front of Pharaoh by the Nile River and Aharon caused all the remaining water of Egypt to turn to blood. Abravanel seems to explain that both Moshe and Aharon struck some water with their staffs, whereas Benno Jacob (1992, p. 256) and Cassuto (1967, p. 98), who follow this idea of dual agents to the plague, explain that only Moshe struck the Nile, while Aharon waived his staff in the air. This latter understanding is more likely, and both actions probably occurred simultaneously: Moshe hit the water in the Nile before Pharaoh, while at the same time, Aharon stretched out his hand and staff in some unknown spot (next to Moshe?) which caused all the remaining water of Egypt to turn to blood.
Accordingly, the second half of 7:17 informs us that Moshe would strike the Nile, while 7:19 records the instructions to Aharon to waive his staff in the air. 7:20 then records that they did as commanded, with the use of the plural referring to both Moshe and Aharon. 7:20 continues and records, "he raised his staff and he hit the water." These words are usually understood as referring to one person. According to the approach that Aharon was the agent of the plague, then the reference is to Aharon (see Altar, 2004, p. 348), while according to the approach that Moshe was the person who hit the water of the Nile, then the reference is to Moshe. With this latter understanding, the statement that “they did” in the beginning of the verse refers to Aharon raising his staff. A different possibility to understanding 7:20 according to the approach that both Aharon and Moshe were agents of the plague is that the words "he raised his staff and he hit the water" are referring to both Moshe and Aharon, with the phrase "he raised his staff" referring to Aharon, and the phrase "he hit the water" referring to Moshe.
Why were there two agents for the plague of blood? Why was not just Moshe or just Aharon the agent of the plague? A reason why Moshe participated in this plague was the need to build up his status with regard to Pharaoh. Previously, Aharon had done the miracle of the staff turning into a reptile and swallowing the staff of Pharaoh’s magicians, 7:10-12, and Aharon was Moshe’s spokesman, 4:14-16; 7;1,2. This might have given the impression that Aharon was the leader of the Jewish people, and if Aharon had also done this plague, then this impression would have only increased. Accordingly, Moshe both spoke directly to Pharaoh and struck the Nile in front of Pharaoh and his advisors, 7:15-18;20, which showed Pharaoh that Moshe was the leader of the Jewish people. These actions show the growth in Moshe that he was able to deal with Pharaoh on his own. On the other hand, Aharon also participated in the plague since the Egyptian magicians, hartumim, were involved in this plague, 7:22, and whenever they appear, Aharon appears as well since both have similar status as being assistants.
With regard to the third questions, the source of water for the Egyptian magicians, many answered have been suggested. The simplest answer is from the Bekhor Shor (on 7:20, quoted by Hizkuni on 7:20) who writes that the water only turned to blood temporarily, and in that brief time period, all the fish died. Bekhor Shor points out that the Torah does not state that the people could not drink the water from the Nile since it was blood, but rather it was the stench of the dying fish that stopped them from drinking the water, 7:18,21. Thus, the Bekhor Shor argues that once the water turned back to water, there was no problem for the magicians to get water since they could have used the water of the Nile River.
One problem with the Bekhor Shor's approach is that if all the blood turned back to water, then all the water that had no fish was drinkable. Yet, 7:24 records that everybody in Egypt had to get water by digging around the Nile. This digging accords with the Bekhor Shor’s idea since this water would not have had any fish, but was there no other water available? Did they not have enough waters in cisterns where there was no fish? The answer is that the Bekhor Shor's explanation only refers to the water that Moshe hit, the Nile River, that this body of water turned back to water almost immediately. However, the water that turned to blood due to Aharon raising his staff remained blood for some time (seven days, 7:25?) and this is the reference to the end of 7:21 "and there was blood throughout Egypt," which relates back to 7:19, the instructions to Aharon. Thus, the Egyptians had to find water that derived from the Nile but did not have fish in it which meant digging for water by the Nile, 7:24. (Note the Bekhor Shor does not follow the dual agent approach to this plague, as in his commentary on 9:8 he writes that Aharon was the sole agent of the plague of blood.)
This understanding of Moshe and Aharon both being agents of the plague of blood coupled with the idea from the Bekhor Shor that the water in the Nile only turned to blood temporarily answers the second question how extensive was the plague. The answer is that all the water in Egypt turned to blood but to a different extent. The water in the Nile River only turned to blood for a very short time, while the other water throughout Egypt turned to blood for a longer period, up to a week.
Bibliography:
Abravanel (1437-1508), 1997, Commentary on Shemot. Jerusalem: Horev.
Alter, Robert, 2004, The five books of Moses: A translation and commentary, New York: W. W. Norton and Company
Cassuto, Umberto (1883-1951), 1967, A commentary on the book of Exodus, Jerusalem: The Magnes Press
Jacob, Benno (1869-1945), 1992, The second book of the Bible: Exodus, translated with an introduction by Walter Jacob, Hoboken: Ktav Publishing House
Rashi (on 7:19) quotes the Midrash (Shemot Rabbah 9:10) that Aharon was the agent of the plague since Moshe could not hit the Nile since the Nile had saved him when he was a baby. Following this approach, Ibn Ezra and Ramban (on 17:5) claim that 17:5 does not imply that Moshe hit the water, but that in 17:5 G-d was telling Moshe to take the stick that Moshe commanded Aharon to hit the Nile. This is not the simple understanding of 17:5.
Abravanel (1997, pp. 105,106) writes that both Moshe and Aharon were the agents of the first plague. Moshe’s blow was in front of Pharaoh by the Nile River and Aharon caused all the remaining water of Egypt to turn to blood. Abravanel seems to explain that both Moshe and Aharon struck some water with their staffs, whereas Benno Jacob (1992, p. 256) and Cassuto (1967, p. 98), who follow this idea of dual agents to the plague, explain that only Moshe struck the Nile, while Aharon waived his staff in the air. This latter understanding is more likely, and both actions probably occurred simultaneously: Moshe hit the water in the Nile before Pharaoh, while at the same time, Aharon stretched out his hand and staff in some unknown spot (next to Moshe?) which caused all the remaining water of Egypt to turn to blood.
Accordingly, the second half of 7:17 informs us that Moshe would strike the Nile, while 7:19 records the instructions to Aharon to waive his staff in the air. 7:20 then records that they did as commanded, with the use of the plural referring to both Moshe and Aharon. 7:20 continues and records, "he raised his staff and he hit the water." These words are usually understood as referring to one person. According to the approach that Aharon was the agent of the plague, then the reference is to Aharon (see Altar, 2004, p. 348), while according to the approach that Moshe was the person who hit the water of the Nile, then the reference is to Moshe. With this latter understanding, the statement that “they did” in the beginning of the verse refers to Aharon raising his staff. A different possibility to understanding 7:20 according to the approach that both Aharon and Moshe were agents of the plague is that the words "he raised his staff and he hit the water" are referring to both Moshe and Aharon, with the phrase "he raised his staff" referring to Aharon, and the phrase "he hit the water" referring to Moshe.
Why were there two agents for the plague of blood? Why was not just Moshe or just Aharon the agent of the plague? A reason why Moshe participated in this plague was the need to build up his status with regard to Pharaoh. Previously, Aharon had done the miracle of the staff turning into a reptile and swallowing the staff of Pharaoh’s magicians, 7:10-12, and Aharon was Moshe’s spokesman, 4:14-16; 7;1,2. This might have given the impression that Aharon was the leader of the Jewish people, and if Aharon had also done this plague, then this impression would have only increased. Accordingly, Moshe both spoke directly to Pharaoh and struck the Nile in front of Pharaoh and his advisors, 7:15-18;20, which showed Pharaoh that Moshe was the leader of the Jewish people. These actions show the growth in Moshe that he was able to deal with Pharaoh on his own. On the other hand, Aharon also participated in the plague since the Egyptian magicians, hartumim, were involved in this plague, 7:22, and whenever they appear, Aharon appears as well since both have similar status as being assistants.
With regard to the third questions, the source of water for the Egyptian magicians, many answered have been suggested. The simplest answer is from the Bekhor Shor (on 7:20, quoted by Hizkuni on 7:20) who writes that the water only turned to blood temporarily, and in that brief time period, all the fish died. Bekhor Shor points out that the Torah does not state that the people could not drink the water from the Nile since it was blood, but rather it was the stench of the dying fish that stopped them from drinking the water, 7:18,21. Thus, the Bekhor Shor argues that once the water turned back to water, there was no problem for the magicians to get water since they could have used the water of the Nile River.
One problem with the Bekhor Shor's approach is that if all the blood turned back to water, then all the water that had no fish was drinkable. Yet, 7:24 records that everybody in Egypt had to get water by digging around the Nile. This digging accords with the Bekhor Shor’s idea since this water would not have had any fish, but was there no other water available? Did they not have enough waters in cisterns where there was no fish? The answer is that the Bekhor Shor's explanation only refers to the water that Moshe hit, the Nile River, that this body of water turned back to water almost immediately. However, the water that turned to blood due to Aharon raising his staff remained blood for some time (seven days, 7:25?) and this is the reference to the end of 7:21 "and there was blood throughout Egypt," which relates back to 7:19, the instructions to Aharon. Thus, the Egyptians had to find water that derived from the Nile but did not have fish in it which meant digging for water by the Nile, 7:24. (Note the Bekhor Shor does not follow the dual agent approach to this plague, as in his commentary on 9:8 he writes that Aharon was the sole agent of the plague of blood.)
This understanding of Moshe and Aharon both being agents of the plague of blood coupled with the idea from the Bekhor Shor that the water in the Nile only turned to blood temporarily answers the second question how extensive was the plague. The answer is that all the water in Egypt turned to blood but to a different extent. The water in the Nile River only turned to blood for a very short time, while the other water throughout Egypt turned to blood for a longer period, up to a week.
Abravanel (1437-1508), 1997, Commentary on Shemot. Jerusalem: Horev.
Alter, Robert, 2004, The five books of Moses: A translation and commentary, New York: W. W. Norton and Company
Cassuto, Umberto (1883-1951), 1967, A commentary on the book of Exodus, Jerusalem: The Magnes Press
Jacob, Benno (1869-1945), 1992, The second book of the Bible: Exodus, translated with an introduction by Walter Jacob, Hoboken: Ktav Publishing House