Even if people know that the presence of water in the car fuel tank damages the car engine, they do not always know how much water does it take to damage an engine. Well, a small amount of water is anyway present in a dissolved state in the fuel that does not cause any harm to the engine. But if the amount of water present is somewhere near a cup or a bit less, there is a problem. So anything more than 0.5% by volume of the fuel tank capacity can be dangerous to the car engine. Beyond this measure, it is fine!