It is possible upon occasion to develop moisture on top of your water mattress. On almost all occasions this occurs in Softside Waterbeds used with the absence of a heater...
When there is a sudden temperature change to colder weather your water temperature will fall. The water temperature stays a few degrees below room temperature...as this happens, your body's temperature is constant...
When the difference between your body temperature and your water temperature are great, this may cause a sweat...(condensation)...
Although this can occur more often in some area's then others, it is rare...All conditions must be right and depend greatly on the time of year...type of floor...and how well made and insulated you keep your mattress...
To keep condensation form ever occurring the remedy is to add a UL approved hybrid waterbed heater.
The softside heater is smaller than a regular waterbed heater, It's designed for use with Softside Waterbeds that use much less water.
If condensation should ever occur.....un-zip the cover to the softside mattress, fold it back and air it out, spray a disinfectant on any moist areas in the cover. a fan works well to dry and kill mold.
Place a towel ( to absorb any moisture ) on the area of the vinyl mattress where the warmest part of your body (back area) would lay. until the temperature is once again stable.