No temp reading

'97 Sport nautique PLL Injected motor. I have a dead temp gauge. I did a bit of research so I could start diagnosing. I found out that the motor has two temp sending units, one for gauge, one for injection. Are these the two units and which is which? "A" "B"
There is also a sending unit (two wire sealed plug) at the top of the motor near the intake manifold and what I think is a water channel. What is this? "C".
I am going to change the impeller and looked up some posts and videos on it. None of the motors I see had a configuration like mine. Many had the water pump on a bracket. Is the thing I circled in Red my pump? If so what is the top pulley for? looks like teh water channel goes there as well?

Is this a Ford or GM based motor?

Thanks,
Adam

It's a Chevy,the pump on the crank is the raw water pump,pumps water out of the lake your in the pulley's run the circulating pump that makes the water flow thru the block. Correct on how to test the gauge,remove wire from sender and ground it and turn the key to run.
Circuit breakers take the place of fuses

I have no idea which sensor is which,I don't have a Chevy. The crank pump is a pump that creates pressure and has a rubber impeller. The excess water from this pump goes to the exhaust all the time. The block water pump just pushes water thru the block but with out much pressure.It has vanes in it made out of brass usually and has no user serviceable parts,They might start to leak or make noise and are just replaced