The sensors are junk. In fact they were so bad that they quit using them in 2000.
Just ignore the light or if you can't a new sensor is around $40 from GM and it's located just below the upper radiator hose. It's a square little box with wires going to it.
As far as running straight water, don't do it for very long. Coolant has lubricants in it for the water pump and corrosion inhibitors and it raises the boiling point way above that of water.
If you want to stay with water as a coolant then get some "Water Wetter" or "Purple Ice" they also have the lubricants and corrosion inhibitors in them and will make your car run a little cooler.

 

Yeah thanks man. I think you got a point re. sensors, cos when there was a leak in the radiator and the car overheated to the max and stoped mid-road last month, the coolant light didnt even show up. But this saturday I will be going to fill in the coolant.

 

yep.. l replaced my radiator and everything is fine now, temp. is fine now but light is still on and l dont bother.. before l put the sensor back on, l messed up a little bit and then l put back together and installed back to the car.. l am 99% sure my sensor is bad and yours too.. so dont bother..

 

Alrightey, the shop guy drained some of the water and topped the tank with coolant. Now i've got a well regulated engine and the coolant light doesnt show anymore. The temp occasionally goes to about 103/104 C which someone on this forum said was normal but not anything above that temp. As always, thanks for the advice everyone.

 

Bingo!!, you still have air in the system, and probably a bad low coolant sensor.

SteveC