I haven't seen it but my guess is they did it the same way they did water and cloth effects on an invisible Kevin Bacon in the movie "Hollow Man"

They shot a person in a solid color suit (chroma key blue or green for some shots, matte black for most of the water shots.) Then in post they can clean out and remove the actor leavingthe water effects behind.

Maybe they painted a car matter black, and dumped the water on it. That's what I'd do.