A vessel having a ballast tank comprising a liquid pump, the apparatus comprising: an immersion assembly comprising at least one immersion member having a liquid conduit provided therein along at least a portion of a length thereof, the immersion assembly being arranged whereby in use at least a portion of the immersion assembly is immersed in liquid to be pumped, characterised in that: the apparatus is arranged to recirculate liquid in the ballast tank, the apparatus being arranged to provide a supply of gaseous fluid to liquid in the tank through the at least one immersion member at one of a plurality of vertically spaced apart locations (131A, 131B) of the assembly thereby to cause passage of liquid through the assembly from a liquid inlet aperture to a liquid outlet aperture of the assembly, the apparatus being configured such that, in use, the location at which gaseous fluid is supplied to the immersion assembly is dependent on the level of liquid in the ballast tank, the apparatus comprising a liquid level monitoring device (141A, 141B), the apparatus being arranged to determine the vertical position at which gaseous fluid is supplied to the liquid conduit based on a level of liquid in the ballast tank. an immersion assembly comprising at least one immersion member having a liquid conduit provided therein along at least a portion of a length thereof, the immersion assembly being arranged whereby in use at least a portion of the immersion assembly is immersed in liquid to be pumped, characterised in that: the apparatus is arranged to recirculate liquid in the ballast tank, the apparatus being arranged to provide a supply of gaseous fluid to liquid in the tank through the at least one immersion member at one of a plurality of vertically spaced apart locations (131A, 131B) of the assembly thereby to cause passage of liquid through the assembly from a liquid inlet aperture to a liquid outlet aperture of the assembly, the apparatus being configured such that, in use, the location at which gaseous fluid is supplied to the immersion assembly is dependent on the level of liquid in the ballast tank, the apparatus comprising a liquid level monitoring device (141A, 141B), the apparatus being arranged to determine the vertical position at which gaseous fluid is supplied to the liquid conduit based on a level of liquid in the ballast tank. the apparatus is arranged to recirculate liquid in the ballast tank, the apparatus being arranged to provide a supply of gaseous fluid to liquid in the tank through the at least one immersion member at one of a plurality of vertically spaced apart locations (131A, 131B) of the assembly thereby to cause passage of liquid through the assembly from a liquid inlet aperture to a liquid outlet aperture of the assembly, the apparatus being configured such that, in use, the location at which gaseous fluid is supplied to the immersion assembly is dependent on the level of liquid in the ballast tank, the apparatus comprising a liquid level monitoring device (141A, 141B), the apparatus being arranged to determine the vertical position at which gaseous fluid is supplied to the liquid conduit based on a level of liquid in the ballast tank.