WordNet (r) 3.0 (2006):

marine animal
    n 1: any of numerous animals inhabiting the sea including e.g.
         fishes and molluscs and many mammals [syn: marine animal,
         marine creature, sea animal, sea creature]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "marine animal":
   Loch Ness monster, alevin, benthon, benthos, cetacean, crustacean,
   dolphin, dugong, fingerling, fish, fry, game fish, grilse,
   harbor seal, kipper, man-eater, man-eating shark, manatee, minnow,
   minny, nekton, octopus, panfish, plankton, porpoise, salmon,
   sea dog, sea lion, sea monster, sea pig, sea serpent, sea snake,
   sea urchin, seal, shark, shellfish, smolt, sponge, squid,
   tropical fish, walrus, whale