And the magicians - did so - But if all the water in Egypt was turned into blood by Moses, where did the magicians get the water which they changed into blood? This question is answered in Exodus 7:24. The Egyptians dug round about the river for water to drink, and it seems that the water obtained by this means was not bloody like that in the river: on this water therefore the magicians might operate. Again, though a general commission was given to Moses, not only to turn the waters of the river (Nile) into blood, but also those of their streams, rivers, ponds, and pools; yet it seems pretty clear from Exodus 7:20 that he did not proceed thus far, at least in the first instance; for it is there stated that only the waters of the river were turned into blood. Afterwards the plague doubtless became general. At the commencement therefore of this plague, the magicians might obtain other water to imitate the miracle; and it would not be difficult for them, by juggling tricks or the assistance of a familiar spirit, (for we must not abandon the possibility of this use), to give it a bloody appearance, a fetid smell, and a bad taste. On either of these grounds there is no contradiction in the Mosaic account, though some have been very studious to find one.

The plague of the bloody waters may be considered as a display of retributive justice against the Egyptians, for the murderous decree which enacted that all the male children of the Israelites should be drowned in that river, the waters of which, so necessary to their support and life, were now rendered not only insalubrious but deadly, by being turned into blood. As it is well known that the Nile was a chief object of Egyptian idolatry, (See Clarke's note on Exodus 7:15), and that annually they sacrificed a girl, or as others say, both a boy and a girl, to this river, in gratitude for the benefits received from it, (Universal Hist., vol. i., p. 178, fol. edit)., God might have designed this plague as a punishment for such cruelty: and the contempt poured upon this object of their adoration, by turning its waters into blood, and rendering them fetid and corrupt, must have had a direct tendency to correct their idolatrous notions, and lead them to acknowledge the power and authority of the true God.

for they

Exodus 7:18-21 And the fish that is in the river shall die, and the river shall stink...

Hope for the Heathen
Throughout his ministry Isaiah bore a plain testimony concerning God's purpose for the heathen. Other prophets had made mention of the divine plan, but their language was not always understood. To Isaiah it was given to make very plain to Judah the truth that among the Israel of God were to be numbered many who were not descendants of Abraham after the flesh. This teaching was not in harmony with the theology of his age, yet he fearlessly proclaimed the messages given him of God and brought hope …
Ellen Gould White—The Story of Prophets and Kings