Abstract: A pump comprises a flexible container (11) filled with a fluid and having a fluid inlet (104) and a fluid outlet (105), and a constricting device (12), preferably in the form of a braid tube, conforming the container (11) to a generally circular cross-section and operable cyclically to reduce the diameter of the container (11) to expel fluid from the outlet and then allow the diameter to increase with fluid entering the inlet to refill the container (11). Such a pump can be suspended in the sea between an anchor and means for converting energy from the sea into a force acting on said constricting device, for example, a float, to cause the constricting device to reduce the diameter of the container (11) to expel fluid from the outlet (105), and then passing the expelled fluid to do work such as drive a generator (121).