The present invention relates to an underwater propelling device (10) i a stator (20) comprising circumferential arrayed coils (35) and being adapted to generate a controlled electromagnetic field along an axial direction, a shaftless propellant rotor (30) comprising: at least two magnetized ring-shaped plates, respectively an upper magnetized plate (31) and a lower magnetized plate (32), coaxially spaced-apart along said axial direction and perpendicular thereto such that said circumferential arrayed coils (35) are positioned therebetween, each plate (31, 32) defining a central aperture (31a, 32a) and having a circumferential array of radially extended magnetized poles (34) embedded therein, said magnetized poles (34) generating a magnetic field which interacts with the electromagnetic field of the stator (20) so as to cause the rotation of said plates (31, 32) about a central axis (ZZ′) parallel to said axial direction, at least one ring-shaped propeller (33) defined by its outer periphery and its inner periphery, said propeller being received inside the central apertures (31a, 32a) of said plates (31, 32) and fixedly secured thereto, a plurality of propeller blades (35) projecting inward from the inner periphery of said propeller (33), said blades (35) producing a thrust along the central axis (ZZ′) when the plates (31, 32) rotate, wherein the rotor (30) is axially separated from the stator (20) by a gap, preferably less than 3 mm, so as to permit the formation of a hydraulic film inside said gap, said hydraulic film being adapted to provide lubrication of the rotor, thus reducing friction between the rotor and the stator, and to transmit the axial thrust force of the rotor to the stator.