Dry Water

Okay, from a strictly chemical point of view, 3M's Novec 1230 isn't the plain old dihydromonoxide we have known of old... but that is a good thing.

This fascinating liquid (video here) evaporates with incredible speed, does not conduct electricity and is apparently non-toxic. Perfect for, say, filling vulnerable voids in a home-made U(nderwater)AV to pressure proof 'em, or having a weirdly dry water fight.