The Collaborative International Dictionary of English v.0.48:

Water feather \Wa"ter feath"er\ Water feather-foil \Wa"ter
feath"er-foil`\ (Bot.)
   The water violet (Hottonia palustris); also, the less showy
   American plant Hottonia inflata.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Hottonia inflata
    n 1: a featherfoil of the eastern United States with submerged
         spongy inflated flower stalks and white flowers [syn:
         water gillyflower, American featherfoil, Hottonia
         inflata]