A watercraft (10,40, 60, 90, 110, 200, 210, 310) is disclosed which is propelled by at least one pair of pivoted sheets (28, 29) which are reciprocated through the water by a propulsion mechanism (18, 25, 45, 48, 91, 92) which is driven by repeated weight transfer of an operator's body (20). Also disclosed is a hull assembly system (61, 161, 261). The water craft disclosed are particularly suited as pleasure craft able to be transported by a vehicle and operated by many classes of persons including those without endurance or stamina.