The Collaborative International Dictionary of English v.0.48:

Cockleshell \Coc"kle*shell`\, n.
   1. One of the shells or valves of a cockle.
      [1913 Webster]

   2. A light boat.
      [1913 Webster]

            To board the cockleshell in those plunding waters.
                                                  --W. Black.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cockleshell
    n 1: a small light flimsy boat