Abstract: A normally flat horizontal blanket-type waterbed heater unit having top and bottom laminates of thin flexible dielectric sheet plastic material and an elongate electric resistance element arranged and sealingly engaged between the laminates. A power supply cord connects with opposite ends of the element and extends from the unit. The unit further includes a normally open safety switch connected with and between related ends of sections of the element. The switch includes spaced terminals atop the bottom laminate, a contact bar above the terminals and below the top laminate and a resilient insulating support part between the laminates and adjacent the terminals and bar. The part normally yieldingly holds the bar and adjacent portion of the top laminate spaced above the terminals. The unit is arranged atop a water bed mattress supporting platform and below the bottom wall of a sheet plastic waterbed mattress bladder.