Waterbeds have their own heating system with a built-in thermostat that should maintain a steady temperature. Once you have the temperature at the level that you like, you should not need to adjust it again.
Step 1 - Identify Comfortable Temperature
The comfort zone of waterbeds is between 85 and 92 degrees Fahrenheit. It is best to start off at the low end of the range and then increase it gradually until you find the ideal level.
Step 2 - Maintain the Temperature
When unoccupied, make the bed up as the extra insulation will help it maintain the water temperature.
Step 3 - Position the Temperature Sensor Away from the Heating Pad
If the heat sensor of your waterbed is located too near the heating pad, the heat sensor will get a false reading from conducted heat in the bed frame.
Step 4 - Understand Electricity Concerns
The cost of the electricity used by a waterbed is relatively low. The water temperature does not fall quickly, but does warm up fairly quickly both from the heater pad and any body heat when occupied. The heater pad is double insulated against the water by the vinyl water bag and the frame cover.
A waterbed will maintain its temperature if it is covered most of the time, has a correctly set thermostat and a temperature sensor positioned some distance away.