1996 Chevrolet Caprice water pump

That light means the coolant level is low in the reservoir. There's no way to detect if coolant is flowing in the cooling system. You can find that out yourself by feeling the radiator hoses and running your hand along the front of the radiator. There shouldn't be any cool spots surrounded by hot areas. Cold spots indicate blockage in the radiator.

I've filled the reservoir but the light is still on. Is it because I filled it with just water instead of a 50/50 mix of ati-freeze?

Sorry for the delay.

Nope, it's only the level, not what's in there. You can unplug the switch if you can find it. If that makes the light go off, the float inside the reservoir is likely stuck.