WordNet (r) 3.0 (2006):

Poseidon
    n 1: (Greek mythology) the god of the sea and earthquakes in
         ancient mythology; brother of Zeus and Hades and Hera;
         identified with Roman Neptune

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "Poseidon":
   AB, Agdistis, Amor, Ancient Mariner, Aphrodite, Apollo, Apollon,
   Ares, Argonaut, Artemis, Ate, Athena, Bacchus, Ceres, Cora, Cronus,
   Cupid, Cybele, Davy, Davy Jones, Demeter, Despoina, Diana,
   Dionysus, Dis, Dylan, Eros, Flying Dutchman, Gaea, Gaia, Ge,
   Great Mother, Hades, Helios, Hephaestus, Hera, Here, Hermes,
   Hestia, Hymen, Hyperion, Jove, Juno, Jupiter, Jupiter Fidius,
   Jupiter Fulgur, Jupiter Optimus Maximus, Jupiter Pluvius,
   Jupiter Tonans, Kore, Kronos, Magna Mater, Mars, Mercury, Minerva,
   Mithras, Momus, Neptune, Nereid, Nereus, Nike, OD, Oceanid,
   Oceanus, Olympians, Olympic gods, Ops, Orcus, Persephassa,
   Persephone, Phoebus, Phoebus Apollo, Pluto, Proserpina, Proserpine,
   Rhea, Saturn, Tellus, Thetis, Triton, Varuna, Venus, Vesta, Vulcan,
   Zeus, able seaman, able-bodied seaman, bluejacket, buccaneer,
   deep-sea man, fair-weather sailor, fisherman, fresh-water nymph,
   hearty, jack, jack afloat, jack-tar, jacky, kelpie, limey, limniad,
   lobsterman, man fish, mariner, matelot, mermaid, merman, naiad,
   navigator, nix, nixie, ocean nymph, pirate, privateer, sailor,
   salt, sea devil, sea dog, sea god, sea nymph, sea rover, sea-maid,
   sea-maiden, seafarer, seafaring man, seaman, shipman, siren, tar,
   undine, viking, water dog, water god, water spirit, water sprite,
   whaler, windjammer, windsailor