Apparatus for converting a bicycle to a maneuverable, pedal-operable aquacycle is disclosed. A twin hull assembly includes a support framework for supporting a bicycle by its horizontal frame member. A friction wheel, which may be rotated with the rear wheel of the bicycle by pedal action, turns symmetrically spaced apart, oppositely pitched, twin propellers via dual flexible axles that extend laterally and rearwardly from the friction wheel. An in-line dual rudder control system is provided that, responsive to the turning of the handlebars, efficiently turns the aquacycle in a direction corresponding to the direction a bicycle would turn. Except for conversion hardware consisting of two U-bolts and four nuts, the apparatus is of lightweight, polymeric materials.