حُرِّمَتْ عَلَيْكُمُ الْمَيْتَةُ
Carrion has been prohibited to you.
This prohibition does not by itself prove that water necessarily becomes impure whenever any animal dies in it. Prohibition of eating carrion and the purity of water are distinct legal questions. Connecting them requires further evidence about the water, the animal, and impurity.
The example of a frog dying in water demonstrates why a text about forbidden consumption cannot independently establish every rule concerning contamination.