All water has a degree of acidity in it, because we've ****ed up the planet. However, he can probably purify the water in himself.
I remember one time he lost his body and was just a dismembered head. The X-Men were being kept in a controlled environment and he couldn't get any moisture to reform. Havok was tired of his bitching and offered to take a piss to give him some raw materials to work with lol.