Here's a well-founded version of the same argument so you can see that the logic actually works:
1) If water does not exist then fish cannot exist. (Well founded P1 because fish obviously require water to survive)
2) Fish exist.
3) Therefor water must exist.
1) If water does not exist then fish cannot exist. (Well founded P1 because fish obviously require water to survive)
2) Fish exist.
3) Therefor water must exist.