An apparatus for loading a powdered electrolyte into a chamber made in a tubular casing of a craft operating in an underwater environment, comprising: a valve carried by the tubular casing, configured to be arranged in a normally closed position and designed to be moved, as a result of manual operation, to an open position to allow the introduction of the powdered electrolyte into the chamber from outside the casing; a feeder-actuator assembly configured to be mounted on the tubular casing to couple with the valve in a fluid-tight manner and enable the opening/closing of the valve; and an electrolyte tank configured to couple with the feeder-actuator assembly and feed powdered electrolyte by gravity to the actuator assembly. The actuator assembly delimits an elongated internal cavity configured to allow a flow of powdered electrolyte from the tank to the chamber through the valve.