וְהַדָּגָ֧ה אֲשֶׁר־בַּיְאֹ֛ר תָּמ֖וּת וּבָאַ֣שׁ הַיְאֹ֑ר וְנִלְא֣וּ מִצְרַ֔יִם לִשְׁתּ֥וֹת מַ֖יִם מִן־הַיְאֹֽר׃ (ס)
And the fish that are in the river shall die, and the river shall become foul; and the Egyptians shall loathe to drink water from the river.’
Rashi on Exodus
ונלאו מצרים THE EGYPTIANS SHALL WEARY THEMSELVES in seeking a cure for the waters of the river that they may become fitted for drinking.
Rashbam on Exodus
ונלאו, I explained the origin of this word already on Genesis 19,11. It is a reinforced version of saying “they could not.” It is an expression of being powerless.
Sforno on Exodus
והדגה אשר ביאור תמות, the river will not retain the appearance of a river, part of it has been turned into blood; but the entire river bed will be filled with blood so that the fish have no habitat in which to survive.