How exactly would a mechanical water temp guage work? Oil guages are mechanical because it uses the pressure of the oil to move the needle against a spring. Since oil is an incompressible fuild, you can run it all the way to the guage without and change in pressure.
Water temperature is detected by electrical means, or else by a bi-metal spring. The electrical sensors are more accurate, and if you actually ran water all the way into the cabin, the temp would drop dramatically, and it would be very innacurate - it would even change accuracy with outside/inside temps.
