A plunger apparatus, comprising: a plunger body able to travel within the tubular string; a flow channel through the plunger body; an actuatable external seal assembly operatively provided on or within an outer surface of the plunger body; and a plug mechanism mechanically integrated with the plunger body and reversibly moveable between a closed position and an open position with respect to the plunger body and flow channel, the open position configured to permit the passage of a continuous water slug past the plug mechanism and through the flow channel; and wherein the actuatable external seal assembly is actuated between the deployed position when the plug mechanism is operatively in the closed position and the retracted position when the plug mechanism is operatively in the open position.