A water injection device of an internal combustion engine is provided. The water injection device includes a water tank for storing water, and a delivery element for delivering the water. The delivery element is connected to the water tank. The water injection device further includes at least one water injector for injecting water. The at least one water injector is connected to the delivery element. The water injection device further includes a pressure sensor, which is arranged in a line region between the delivery element and the water injector, and a control unit, which is designed to determine, on the basis of pressure data of the pressure sensor, a formation of vapor in the aforementioned line region when the water injector is closed and the delivery element is deactivated.