A boat has installed therein a primary propulsion system to produce a thrust at least in the longitudinal direction, manual controls for controlling the primary propulsion system, a transverse propulsion system for producing a transverse thrust, and a joystick control for controlling the primary and transverse propulsion systems for navigation at slow speed or in docking manoeuvres. An electronic control unit controls the propulsion systems and receives information on the instantaneous speed of the boat and on the instantaneous distance between the boat and an obstacle such as the coast, the bottom or floating or half-submerged bodies. The electronic unit is so arranged that when the boat is located at a distance from an obstacle less than a predetermined distance it controls the propulsion systems to reduce the speed of the boat to below a predetermined limit, disables the manual controls of the primary propulsion system and enables the joystick control.