Rashi on Exodus 4:9/רש"י על שמות ד׳:ט׳

2והיו המים וגו׳ — Here we have the word והיו twice. It appears to me that if it was stated היאר לדם ביבשת מן והיו המים אשר תקח, I might understand it to mean that it would be turned into blood in his hand and that also when it reached the ground it would retain that state (more lit., it would be as it was). But now (as the text stands) it tells us that it would not become blood until it would come on the dry ground.