Contactless reed switches from Littelfuse provides flow and water presence monitoring. Link here (PDF)
Reed switches and sensors are highly effective solutions for flow sensing applications that can be used for detecting the presence of fluid flow in a system or even measure the rate of fluid flow.
Tankless water heaters are becoming a widely popular solution for water heating needs. The availability of hot water on demand and the perceived limitless supply of hot water make these types of water heaters much more attractive than traditional tank water heaters. These water heaters also provide long-term energy savings since energy is used only when there is a demand for hot water. In order to effectively heat the water when there is a demand, a sensor is needed to detect the flow of water.