A surf ski is a long, narrow, lightweight kayak

Kayak

A kayak is a small, relatively narrow, human-powered boat primarily designed to be manually propelled by means of a double blade paddle.The traditional kayak has a covered deck and one or more cockpits, each seating one paddler...

 with an open (sit-on-top) cockpit, usually with a foot pedal controlled rudder

Rudder

A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...

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Typically 5-6.5m (16½-21ft) long and only 40-50 cm (16-20") wide, surf skis are extremely fast when paddled on flat water (only an Olympic K1 or K2 kayak is faster) and the fastest paddled craft available over a long distance on ocean swells. They track well but are less maneuverable and have less transverse primary and secondary stability than shorter, wider craft.