Since this is a mashal, then it is not to be taken literally. The mashal of water and the nimshal of Torah is what is important. Just as originally before the Jews received the Torah they lived as Bnei Noach, those who are attempting to destroy the Bnei Yisrael are saying that the Jews should abandon the Torah and assimilate with the nations of the world. The Torah is essential to the existence of Jews as air is essential to animals and water is essential to fish. Attempting to assimilate with the nations is as deadly to us as air is to a fish.
One can say that since the Jews were Bnai Noach before the giving of the Torah, that the fish lived without water originally. That is since before Avraham Avinu one can claim that the ancestors of the Jews were idol worshipers the fox (the idol worshipers of the current day) is now attempting to pretend that the Jews would be able to survive without the Torah (a false claim).