An automotive water heater having application to a windshield of an automobile, which is able to produce hot water that can be sprinkled on the windshield to melt accumulated snow and frost, in which a water heater is mainly structured from a laminated assembled heating unit side joined to a heat conducting surface of an exchange bed. Heat quantity produced by the heating unit forms a heat balance function, which transfers the heat to the exchange bed, and water flow paths are defined interior of the exchange bed. The face joining of the laminated heating units is used to facilitate maintenance of component members and allow changing of the number of heating units assembled to the exchange bed, thereby providing choice to accommodate different power requirements.