Water Pressure Gauge

[Sstgermain]

I have a new to me 2002 Party Barge 240. The motor is a Mercury Magnum 40. The water pressure gauge is a 0 to 30 psi and it barely moves. Previous owner said he had a bad gauge and replaced it with too high a reading gauge. There is water circulating so that is not the problem. Does anyone know the correct pressure and the correct gauge for this boat and motor.

Thanks for your help.