Cats and dogs. Oil and water. Capulets and Montagues. Some things just do not mix.
In sex comedies like Lysistrata, the two things that don't mix are male and female viewpoints. The women want to give peace a chance and the men want to keep war keeping on. The women want national unity and the men want to keep kicking Sparta/Athens' butt. The men want sex and the women want… well, they want sex too. But just not quite as badly as they want peace.
The epic fire/water battle between the Chorus of Old Men and the Chorus of Old Women symbolizes this battle between the sexes:
Fly, fly, Nicodice,
before Calyce and Critylla go up
in flames, fanned all around
by nasty winds
and old men who mean death!
I've just come from the well with my pitcher;
I hoisted it onto my head, and to aid the women,
my fellow citizens faced with fire,
I am with water! (321-335)
Have you ever heard the expression "to fight fire with fire?" This means to give back exactly what is being dished out: if someone insults you, you insult them back. If someone punches you, you punch back.
Well, the women of Greece are definitely not fighting fire with fire. They're fighting fire with good old H2O. They're combating never-ending war with a peaceful protest. They're fighting men's traditionally "fiery passions" (yup, that's intentional)—fighting and gettin' it on—with women's traditional placid, cooling influence.
Compounding this symbol some super-sexual imagery. Men's sexual frustration is often described as "raging," "hot," "burning," "smoldering," and so on. And sure, women's passions are also referred to in these terms, but the adjective that comes up first and foremost in describing female arousal is—like water—wet.