Steering and/or stabilising device for motorised watercraft, with at least two operating units comprising each a retractable fin (4), characterised in that the retractable fins of each operating unit are contained in tubular watertight casings (5) open on the bottom or on the underwater area of the hull sides and located completely or for their prevailing portion inside the hull, one on the starboard side and the other on the port side, each fin (4) being guided axially in the associated tubular casing (5) and being movable by an actuator (10) outwards and inwards in the casing (5), so that it can be retracted in the casing (5) or projected out of the hull for a variable extent in the water.