Without a heater the waterbed will assume the temperature of the room - 2 º,  in most cases 65 to 75 º ; that's at least twenty degrees below your body temperature. Since the water can absorb a great deal of body heat a unheated waterbed will feel very cold. The temperature must be brought up to within a few degrees of body temperature in order to be comfortable and relaxing. Some softside waterbeds use insulating mattress toppers that eliminate the use of a heater.

Do I really need a heater?