A boat water pump may produce 150 PSI in a closed system, but it won't come close to maintaining that pressure once you start to draw water through it. It won't make a better wash-down pump than a pump sold for the purpose and it certainly won't maintain a higher pressure in a home or building.
The pump has a pressure switch that turns it on when the pressure gets too low (possibly 15 PSI) and turns it off when the pressure gets to the pressure the system is designed for (possibly 30 PSI in a boat). Either the switch has failed, or it was accidentally bypassed when installing the pump. The switch is a service part and can be replaced.
