The present invention relates to a watertight apparatus for an underwater motor, comprising a double oil jacket for preventing high-pressure water from entering the underwater motor along a rotary shaft of the underwater motor. According to the present invention, high-pressure water is prevented from entering the underwater motor along the rotary shaft of the underwater motor even in the case of mechanical devices which operate in deep water, thereby preventing failure in the operation of or damage to mechanical devices caused by the ingress of water. In addition, the oil jacket can be easily replaced, thereby enabling the effective performance of maintenance and repair of mechanical devices.