The idea that Water is necessarily H20, is denied by many philosophers. Thanks to Quine, it seems that we can reduce all propositions to synthetic propositions, even the proposition that, all bachelors are unmarried men.
The debate here is between Willard Van Orman Quine, and Saul Kripke.
If we want to allow for analytic propositions, then we can in fact argue that it is logically impossible for a Prime Minister to be Prime Number. What is a Prime Minister, and what are prime numbers? There can be no meaningful proposition, unless the words contained therein have meaning and intension. A prime minister, and a prime number, are not merely objects named, but also objects described, by their respective definitions.