The plague has a smell. Targum Pseudo-Jonathan on Exodus 7:21 stays close to the Hebrew, but what it describes is the sensory aftermath of a cosmic blow. The fish that were in the river died; and the river became foul, and the Mizraee could not drink of the waters.
This is the second layer of the first plague. Blood by itself is a color change. What makes it unbearable is the collapse of the ecosystem. The Nile, home to the sacred fish Egypt revered, is suddenly a graveyard of floating corpses. The stench rises over the delta. Every well and trench carries it too, and the cattle refuse to drink.
The meturgeman adds nothing mystical here. He does not need to. The horror is in the detail: the Mizraee could not drink. An empire that had ruled through the mastery of water could not, for a full week, fill a cup. Pharaoh's chefs, his priests, his children — all stood thirsty in front of the greatest river of the ancient world.
The takeaway is visceral and plain. Idolatry always eventually stinks. When the river you worshipped rots in front of you, the question is no longer whether the God of Israel exists; the question is how long you can hold your breath before admitting it.