An apparatus and method for portably heating water for the purpose of showering or cleanup. A flow of water, originating from a supply container, is warmed as it passes through a heat exchanger transferring energy from a heat source. The heated flow of water is deposited into an accumulating container. The water temperature is actively and automatically controlled by a temperature-responsive valve. Responding to the temperature of the flowing water, the temperature-responsive valve varies the flow of water so as to produce an accurate water temperature within the accumulating container. The flow of water is gravitationally motivated by a pressure head differential between the supply container positioned at an upper elevation and the accumulating container positioned at a lower elevation.