99GTvert

Low coolant light comes on while I'm driving but there is still coolant in the overflow tank. It was below the line when the engine was cold, so I added water (need to get some green coolant eventually), and its still coming on.

Could the sensor have gone bad, or could the water/coolant mixture be too watery to the point where the sensor is not sensing coolant?

Camster

Probably a bad sensor(possibly integral with the tank,requiring the whole thing)
Water/anti freeze dilution has nothing to do with it.It senses liquid.
If the level dropped,look for a manifold leak issue,keep a close eye on the level.