http://playground.arduino.cc/ComponentLib/Thermistor
does not mention if this is a ntc or ptc.
the bottom line is the resistance is a curve, not a straight line.
either look-up or calculate.
if high accuracy is needed, calibration is required
as raschemmel mentioned, ice water is 0°C and boiling water is 100°C at sea level, at 29.92inhg barometric pressure
that is not tap water, but distilled water. contaminants can alter the properties of the fluid, but the principle is sound.