Abstract: A boat comprising a tread drive system including pedals which are adjustable to the size of the operator. An axle extends through the bottom of the boat at the center of the tread system and is connected for oscillating movement therewith, the axle carrying at its lower end a horizontal swing arm. A vertical fin is mounted on the free end of the swing arm for pivotal movement relative thereto. The fin is a flat rigid plate which is tiltable between angular stops. The fin widens rearwardly and is confined by top and bottom baffles. The fin is arranged beneath the center area of the boat between an aft stabilizing fin and a front fin acting as a rudder whose pivot bearing may be coaxial with that of the tiltable fin. The aft stabilizing fin corresponds in size and shape to the rudder and is rigidly fastened to the bottom of the boat near its stern, preferably underneath the seat area for the operator. The boat is steered by a horizontal tiller or steering wheel joined to the upper end of the rudder shaft.