Agree with numanoid about animated material. By the past I used an alpha plane in front of the camera (linked to the camera too) with a kind of water material. Default water material at the begining, then I changed some parameters like colors, shadow, etc.And I animated that material , along, X, Y or Z...in fact can't remember exactly. But that was working. Of course renders could be more longer. For the little story, now I prefer to make heat effect in poswork (Afetr Effect); more fast.