Abstract: A submersible, hydraulically-driven pump rotating about a vertical axis, the pump having a short shaft between the hydraulic motor and the impeller of the pump. A cofferdam is placed around the hydraulic pipe and the hydraulic motor, extending from the pump housing to up above the cargo level, with a shaft sealing arrangement being provided between the motor and impeller. The cofferdam is formed as three consecutive chambers around the shaft, extending between the hydraulic motor and the impeller, whereby the upper, first chamber is a receptacle for oil leakage from the hydraulic motor, and the next, second chamber contains a sealing liquid under pressure and is sealed at the top and at the bottom by respective mechanical shaft seals against the upper, first chamber and the lower, third chamber, respectively.