1999 Mariner 200 hp efi low oil indicatior light keeps sounding

I first had the problem when I took it out and the motor would not get above 3000 rpm and the light was blinking with a beeping noise. I have primed the tank on the motor by opening the cap and let it fill while running. I could measure between 5 and 6 volts in the wire off the sending unit. I can also measure 5 to 6 volts off the sending unit on the oil pump gear. I understand if water in the fuel is a problem the little red light at the front of the motor would come on. I could not see it light up but I changed the filter anyway and hooked up a small fresh tank of premixed gas with no positive result. Where to look next?

I want to also add that the constant beeping now begins as soon as the key is turned on. Since the oil pump's indicator is not activated till the motor is running that rules out that possibility. I'm starting to wonder about the oil sensor sending unit. Is there a way to test without expensive equipment?