Abstract: A snowmobile powered watercraft which includes a pair of forward pontoons mounted to the front suspension in place of conventional skis and an U-shaped rear pontoon mounted to the aft portion of the snowmobile body. Propulsion is provided by the endless track assembly of the snowmobile which extends in part beneath the rear pontoon. Also disclosed is a structural assembly for converting the snowmobile into a watercraft which includes the forward and rear pontoons.