A self-propelled apparatus for cleaning the submerged bottom surfaces of a pool or tank in a predetermined regular pattern includes a reversible water jet drive means for propelling the apparatus in opposite directions corresponding to the longitudinal axis of the apparatus. The direction of the discharge of the propelling water jet is changed by mechanical sensors, electrical sensors, or by interrupting the water pump. The source of the pressurized jet stream can include a pump integral to the apparatus or an external portable pump and/or a pump associated with the pool's filtering and/or circulation system.