Abstract: A surfboard type boat convertible into a sailboat or buoy and comprising a circular floating board enclosed by a shell of polyethylene and containing foamed polystyrene. A cone-shaped keel likewise filled with foamed polystyrene is provided on the lower side of the floating board. A hollow cylinder extends upwardly from and perpendicularly to the floating board and a piston provided with a handlebar at its upper end is slidably mounted within the cylinder. A water ejection tube extends transversely of the lower end of the cylinder through the cone-shaped keel. A rudder operable by the heels of the user of the boat is mounted downstream of the outlet of the water ejection tube. By lifting the piston water is sucked into the cylinder and by vigorously pushing down the piston the water is expelled through the ejection tube and the boat is propelled. The piston can be replaced by a mast for a sail without boom operable by the user.