The Collaborative International Dictionary of English v.0.48:

Founder \Found"er\, v. i. [imp. & p. p. Foundered; p. pr. &
   vb. n. Foundering.] [OF. fondrer to fall in, cf. F.
   s'effondrer, fr. fond bottom, L. fundus. See Found to
   establish.]
   1. (Naut.) To become filled with water, and sink, as a ship.
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   2. To fall; to stumble and go lame, as a horse.
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            For which his horse fear['e] gan to turn,
            And leep aside, and foundrede as he leep. --Chaucer.
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   3. To fail; to miscarry. "All his tricks founder." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

foundering
    n 1: (of a ship) sinking [syn: foundering, going under]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "foundering":
   collapsing, deciduous, declining, declivitous, decurrent,
   descendant, descending, down, down-reaching, downcoming,
   downfalling, downgoing, downhill, downsinking, downward, drooping,
   dropping, falling, on the descendant, on the downgrade, plummeting,
   plunging, sagging, setting, sinking, submerging, subsiding,
   tottering, tumbledown