וְהַדָּגָ֧ה אֲשֶׁר־בַּיְאֹ֛ר תָּמ֖וּת וּבָאַ֣שׁ הַיְאֹ֑ר וְנִלְא֣וּ מִצְרַ֔יִם לִשְׁתּ֥וֹת מַ֖יִם מִן־הַיְאֹֽר׃ (ס)
하수의 고기가 죽고 그 물에서는 악취가 나리니 애굽 사람들이 그 물 마시기를 싫어하리라 하라
Rashi
ונלאו מצרים THE EGYPTIANS SHALL WEARY THEMSELVES in seeking a cure for the waters of the river that they may become fitted for drinking.
Da'at Zekenim
ובאש, “and it will become foul smelling;” if it had not turned evil smelling, the Egyptians might not have desisted from trying to drink its waters. The fact that the waters had assumed the colour of blood would not have been enough to discourage them. We witness people drinking the waters of polluted rivers nowadays, as long as the waters have not become putrid. The fish in the river Nile had to die in order to ensure that the river became putrid. Our author wonders why in light of this the name of the plague is not באש, “stench,” instead of דם, “blood.” He concludes that the reason this is so is that stench is not something visible, and it was important that the plague be something visible to everyone’s eyes.
Rashbam
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.