The Collaborative International Dictionary of English v.0.48:

Morass \Mo*rass"\, n. [OE. marras, mareis (perh. through D.
   moeras), fr. F. marais, prob. from L. mare sea, in LL., any
   body of water; but perh. influenced by some German word. See
   Mere a lake, and cf. Marsh.]
   A tract of soft, wet ground; a marsh; a fen.
   [1913 Webster]

   Morass ore. (Min.) See Bog ore, under Bog.
      [1913 Webster]

WordNet (r) 3.0 (2006):

morass
    n 1: a soft wet area of low-lying land that sinks underfoot
         [syn: mire, quagmire, quag, morass, slack]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "morass":
   anarchy, baygall, bind, bog, bottom, bottomland, bottoms,
   buffalo wallow, chaos, clutch, complication, confusion, crunch,
   dunghill, embarrassing position, embarrassment, entanglement,
   everglade, fabulous formless darkness, fen, fenland,
   fine how-do-you-do, foul-up, glade, hassle, hell to pay, hobble,
   hog wallow, holm, hot water, how-do-you-do, imbroglio, jam, jungle,
   knot, labyrinth, license, marais, marish, marsh, marshland, meadow,
   mere, mesh, mess, mire, misrule, mix, mix-up, moor, moorland, moss,
   mud, mud flat, muddle, parlous straits, pass, peat bog, pickle,
   pinch, plight, predicament, pretty pass, pretty pickle,
   pretty predicament, quag, quagmire, quicksand, salt marsh, scrape,
   screw-up, skein, slob land, slough, snafu, snarl, sough, spot,
   squeeze, stew, sticky wicket, strait, straits, sump, swale, swamp,
   swampland, taiga, tangle, tight spot, tight squeeze, tightrope,
   tricky spot, unholy mess, view, wallow, wash, web