Distilled or demineralized water should be used.
Even though calculation is possible with water temperatures from 0-100°C / 32-212°F, the volume determination should still be done as close to 20°C / 68°F as possible.
Since almost no one has an absolutely accurate indicating thermometer, an additional "thermometer error" can be specified. This can be determined with the help of the Thermometer Error calculator. This does not apply here however to the calibration temperature. So 20°C desired calibration temperature means real 20°C. But it is valid for the measured temperature of the water. So the temperature entered there is the display value of the thermometer.

General Information about Pycnometers