nuphar luteum

from The Collaborative International Dictionary of English v.0.48

Candock \Can"dock\n. [Prob. fr. can + dock (the plant). Cf. G.
   kannenkraut horsetail, lit. "canweed."] (Bot.)
   A plant or weed that grows in rivers; a species of
   {Equisetum}; also, the yellow frog lily ({Nuphar luteum}).
   [1913 Webster]