The Collaborative International Dictionary of English v.0.48:

Water crowfoot \Wa"ter crow"foot`\ (Bot.)
   An aquatic kind of buttercup (Ranunculus aquatilis), used
   as food for cattle in parts of England.
   [1913 Webster]

   Great water crowfoot, an American water plant (Ranunculus
      multifidus), having deep yellow flowers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Ranunculus aquatilis
    n 1: plant of ponds and slow streams having submerged and
         floating leaves and white flowers; Europe and North America
         [syn: water crowfoot, water buttercup, Ranunculus
         aquatilis]