Surveys show that most waterbed customers consider comfort the most important factor in choosing a waterbed, enhanced as it is by pleasant, customizable temperature.  This thermostatically controlled temperature is made possible by the waterbed's heater.  The heater has two main elements: the heating pad under the mattress which conveys heat, and the controller which enables us to set the temperature.  With practical insulation (comforters, quilts, etc.) its use has little effect on our electric bill, in fact a warm waterbed allows us to save on heating our bedroom in the winter.  In the summer the flipside is true since during heat waves the waterbed's considerable mass of water, now unheated, has a cooling effect on our warm bodies.