The Collaborative International Dictionary of English v.0.48:

Boatable \Boat"a*ble\, a.
   1. Such as can be transported in a boat.
      [1913 Webster]

   2. Navigable for boats, or small river craft.
      [1913 Webster]

            The boatable waters of the Alleghany. --J. Morse.
      [1913 Webster]