A submersible pump assembly for transference of a fluid medium with low viscosity is disclosed. In one embodiment, the submersible pump assembly includes a cylinder block having cylinders and pistons. A drive shaft is rotatably supported in the cylinder block and coupled to a drive unit. An inclined leading plate is coupled to the pistons and the drive shaft such that the pistons are configured to be axially driven in a reciprocating motion within the cylinders upon rotation of the inclined leading plate. A suction chamber and a pressure chamber are each located in fluid communication with the cylinders. In one operational mode, the fluid medium is transferred from the suction chamber to the pressure chamber during the reciprocating motion of the pistons, when the pistons are active. In another operational mode, the fluid medium is circulated through the suction chamber.