Is it illegal to swim in a public fountain?

even if it was not illegal, i don't think you should be doing it.

our legal system is common law, so anything that's not forbidden is theoretically legal, however like men wearing skirts, it's frown upon by society at large even though it's legal, those are called norms

It is common in Europe and sometimes in the US to swim in your underwear or nude in a public fountain.

It is not illegal to swim in underwear or nude in public fountain.