WordNet (r) 3.0 (2006):

mud flat
    n 1: a tract of low muddy land near an estuary; covered at high
         tide and exposed at low tide

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "mud flat":
   baygall, bog, bottom, bottomland, bottoms, buffalo wallow,
   everglade, fen, fenland, glade, hog wallow, holm, marais, marish,
   marsh, marshland, meadow, mere, mire, moor, moorland, morass, moss,
   mud, peat bog, quagmire, quicksand, salt marsh, slob land, slough,
   sough, sump, swale, swamp, swampland, taiga, wallow, wash