Source : Webster's Revised Unabridged Dictionary (1913)

Cockleshell \Coc"kle*shell`\, n.
   1. One of the shells or valves of a cockle.

   2. A light boat.

            To board the cockleshell in those plunding waters.
                                                  --W. Black.

Source : WordNet®

cockleshell
     n : a small light flimsy boat