Definitions of fin:

• noun:   organ of locomotion and balance in fishes and some other aquatic animals
• noun:   a stabilizer that resembles the fins of a fish
• noun:   the cardinal number that is the sum of four and one
• noun:   a shoe for swimming; the paddle-like front is an aid in swimming (especially underwater)
• noun:   one of a set of parallel slats in a door or window to admit air and reject rain
• noun:   one of a pair of decorations projecting above the rear fenders of an automobile
• verb:   equip (a car) with fins
• verb:   show the fins above the water while swimming

  Example: "The sharks were finning near the surface"

• verb:   propel oneself through the water in a finning motion