The Collaborative International Dictionary of English v.0.48:

lagoon \la*goon"\ (l[.a]*g[=oo]n"), n. [It. or Sp. laguna, L.
   lacuna ditch, pool, pond, lacus lake. See Lake, and cf.
   Lacuna.] [Written also lagune.]
   1. A shallow sound, channel, pond, or lake, especially one
      into which the sea flows; as, the lagoons of Venice.
      [1913 Webster]

   2. A lake in a coral island, often occupying a large portion
      of its area, and usually communicating with the sea. See
      Atoll.
      [1913 Webster]

   Lagoon island, a coral island consisting of a narrow reef
      encircling a lagoon.
      [1913 Webster] Lagophthalmia

WordNet (r) 3.0 (2006):

lagoon
    n 1: a body of water cut off from a larger body by a reef of
         sand or coral [syn: lagoon, laguna, lagune]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "lagoon":
   artificial lake, bayou lake, cistern, dam, dead water, dike, etang,
   farm pond, fishpond, freshwater lake, glacial lake, inland sea,
   laguna, lake, lakelet, landlocked water, linn, loch, lough, mere,
   millpond, millpool, nyanza, oxbow lake, plash, pond, pondlet, pool,
   puddle, reservoir, salina, salt pond, stagnant water,
   standing water, still water, sump, tank, tarn, tidal pond,
   volcanic lake, water hole, water pocket, well