What's the humidity of water?
Humidity is a measure of the amount of water vapor in the air. Given that definition, you cannot measure humidity in water itself. Even at 100% relative humidity, air is still air. It has not become water. It just means that the air is holding the most amount of water it can possibly hold.
Humidity is only a measure of the amount of water vapor in the AIR. Relative humidity measures the amount of water in the air in relation to the maximum amount of water vapor (moisture), in AIR. With a relative humidity of 100%, it will rain. So, in essence, Water has a relative humidity of 100%.