Or HaChaim on Exodus 7:21/אור החיים על שמות ז׳:כ״א

1 והדגה…מתה, and the fish…died; the Torah reports this detail to show that the Egyptians realised that the plague was not just a sleight of hand, i.e. make-believe. The blood would kill the people who drank it (in quantities). The fact that the river stank proved that the plague was for real. When the Torah adds ויהי הדם בכל ארץ מצרים, "the blood persisted throughout the land of Egypt," the reference is to bath-houses, bathtubs, etc. Perhaps the Torah wanted to tell us that the Egyptians took the "blood" from one location to another. If the "blood" had been the result of witch-craft it would have reverted to water as soon as it had been removed from the sphere of the sorcerer who had performed the trick (compare Zohar volume 2 page 192). The Torah tells us that the Egyptians realised that this phenomenon was something over and above the kind they were used to see their own sorcerers perform.