A system for performing flushing through a cooling water pathway of a marine propulsion device (1a to 1c) by water supplied from a water source, the system comprising: a water control device connected to the water source and the cooling water pathway of the marine propulsion device (1a to 1c), the water control device being configured to control a supply of the water from the water source to the cooling water pathway; and a controller configured to control the water control device, cause the water control device to supply the water from the water source to the cooling water pathway so as to perform the flushing, obtain propulsion device data including at least one of a pressure of the water, a flow rate of the water and a concentration of salt contained in the water in the cooling water pathway, and determine whether or not to stop the supply of the water by the water control device based on the propulsion device data, wherein the propulsion device data includes either the pressure of the water or the flow rate of the water in the cooling water pathway, and the controller is further configured to stop the supply of the water by the water control device after elapse of a predetermined period of time since a point of time that either the pressure of the water or the flow rate of the water in the cooling water pathway has become greater than a predetermined threshold, characterized in that the marine propulsion device (1a to 1c) is an outboard motor comprising an engine, wherein the cooling water pathway is provided inside the engine. a water control device connected to the water source and the cooling water pathway of the marine propulsion device (1a to 1c), the water control device being configured to control a supply of the water from the water source to the cooling water pathway; and a controller configured to control the water control device, cause the water control device to supply the water from the water source to the cooling water pathway so as to perform the flushing, obtain propulsion device data including at least one of a pressure of the water, a flow rate of the water and a concentration of salt contained in the water in the cooling water pathway, and determine whether or not to stop the supply of the water by the water control device based on the propulsion device data, wherein control the water control device, cause the water control device to supply the water from the water source to the cooling water pathway so as to perform the flushing, obtain propulsion device data including at least one of a pressure of the water, a flow rate of the water and a concentration of salt contained in the water in the cooling water pathway, and the propulsion device data includes either the pressure of the water or the flow rate of the water in the cooling water pathway, and the controller is further configured to stop the supply of the water by the water control device after elapse of a predetermined period of time since a point of time that either the pressure of the water or the flow rate of the water in the cooling water pathway has become greater than a predetermined threshold, characterized in that the marine propulsion device (1a to 1c) is an outboard motor comprising an engine, wherein the cooling water pathway is provided inside the engine.