An in-line water heater utilizing electrically conductive polymer structures for electrodes. The area of electrodes that confront one another can be varied, and thereby the temperature to which the water is heated can be variably adjusted. The heat is not generated by the electrodes, but instead by the resistance of the water to the electrical current flowing between them. While the electrodes can be moved relative to one another, preferably they will be fixed, and an non-electrically conductive current gating plate can adjustably be placed between them to variably adjust the amount of confronting areas. A field obstructor can be provided at the inlet and outlet of the heater housing to prevent the exit of electrical current from the heater electrodes, and also non-conductive grounding screens in place for secondary safety.