A first stage pressure regulator is provided. A valve body has an inlet and an outlet that define a pressure chamber therebetween. The valve body defines a pressure compensation chamber having an opening fluidly communicating the pressure compensation chamber with the surrounding water. The first stage pressure regulator comprises an inlet tubular union removably received into the inlet. A removable high pressure orifice body defines an orifice therethrough. The orifice body is carried by the valve body proximate the inlet. A valve seat is within the valve body.