Albert Barnes Commentary Exodus 7:18

"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." — Exodus 7:18 (ASV)

Shall lothe - The water of the Nile has always been regarded by the Egyptians as a blessing unique to their land. It is the only pure and wholesome water in their country, since the water in wells and cisterns is unwholesome, while rain water seldom falls, and fountains are extremely rare.