A water-inflow prevention device is provided by installing a closing lid 6 for blocking a duct 2 of an air pressure regulating valve 1, which regulates air pressure inside a cabin 101. A buoyant body 9 is attached to the disc-shaped closing lid 6 installed to the duct 2 so as to be rotatable around a rotation shaft 7. The buoyant body 9 receives a buoyant force from water when immersed in the water. Since the side of the closing lid 6 in which the buoyant body 9 is attached is thereby floated in the water, the closing lid 6 automatically rotates around the rotation shaft 7 so as to close a ventilation port 2a.

A pressure compensation system is provided. A subsea enclosure of the subsea device encloses the chamber. A first pressure compensator has a first compensation volume and provides pressure balancing between ambient medium surrounding the subsea device and the first compensation volume. A first biasing device is configured to bias the first pressure compensator such that the pressure in the first compensation volume is higher than the pressure in the ambient medium surrounding the subsea device. A second pressure compensator has a second compensation volume and provides pressure balancing between the ambient medium and the second compensation volume. A second biasing device biases the second pressure compensator such that the pressure in the second compensation volume is higher than the pressure in the ambient medium. A control unit is connected to control first and second valves arranged in flow connections between the first and second compensation volumes and the chamber.

A water-inflow prevention device is provided by installing a closing lid 6 for blocking a duct 2 of an air pressure regulating valve 1, which regulates air pressure inside a cabin 101. A buoyant body 9 is attached to the disc-shaped closing lid 6 installed to the duct 2 so as to be rotatable around a rotation shaft 7. The buoyant body 9 receives a buoyant force from water when immersed in the water. Since the side of the closing lid 6 in which the buoyant body 9 is attached is thereby floated in the water, the closing lid 6 automatically rotates around the rotation shaft 7 so as to close a ventilation port 2a.