OK. Seems that I’m the only one in the know (been a first hand witnesses, no, I actually participated)
The first makko was “dam”, blood, which affected their water supply. As the Torah writes, they tried unsuccessfully to dig. Medrashim are full with the account that the Jews had no problem whatsoever, even when both were drinking from the same source. Furthermore, it was one of the Jews’ sources of wealth, for if an Egyptian paid a Jew, it would remain water.
The Torah writes an additional passuk, that the fish would also die. That is discussed by Rishonim in different approaches, and also for the need the Torah had to inform of that detail.
The second makko was “tzefardea” (Froggies!!!). Nothing to do with their water supply.