Fire and water is the most intense and delicate combination: one's passion with the other's emotional depth. Handled with care, there's steam — serious chemistry. Otherwise one feels extinguished and the other boiled. It all comes down to handling strong emotions.
One launches (cardinal), the other adapts (mutable): natural flexibility — few head-on collisions. Just don't let direction always come from the same person; the mutable one needs to be heard too.