הָפַ֣ךְ אֶת־מֵימֵיהֶ֣ם לְדָ֑ם וַ֝יָּ֗מֶת אֶת־דְּגָתָֽם׃
Obrócił ich wody w krew, i uśmiercił ryby ich.
Sforno
ויהפוך, the earth as well as its inhabitants underwent a total chemical transformation being turned into brimstone and fire, similar to what David means in Psalms 105,29 “He turned their waters into blood.” The dew that rose from the earth prior to sunrise turned into salt. Compare Deut. 29,22 גפרית ומלח שרפה כל ארצה, meaning that the moisture descended back onto earth but this time mixed with solid burned particles.
Rashbam
(1) ANY OF THE DISEASES THAT I BROUGHT UPON THE EGYPTIANS. That I turned their waters into blood (Ps. 105:29), and they did not have water to drink.