A leak detecting device for water filled vessel such as a swimming pool. An annular resilient seal having a central opening contacts the surface of the vessel under water and creates a seal between the resilient seal and the vessel. A rigid housing open in its interior is coupled to the resilient seal and has an opening for an inlet to deliver fluid into the interior of the housing, or couple a flow meter thereto to measure the fluid flow, or deliver a dye into the housing and through the central opening of the resilient seal.