Charles Ellicott Commentary Exodus 7:21

"And the fish that were in the river died; and the river became foul, and the Egyptians could not drink water from the river; and the blood was throughout all the land of Egypt." — Exodus 7:21 (ASV)

The Egyptians could not drink. — Previously they had “loathed to drink” (Exodus 7:18), but apparently had drunk. Now they could do so no longer — the drink was too nauseous.