The Collaborative International Dictionary of English v.0.48:

Sea dog \Sea" dog`\
   1. (Zool.) The dogfish.
      (b) The common seal.
          [1913 Webster]

   2. An old sailor; a salt. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Seal \Seal\ (s[=e]l), n. [OE. sele, AS. seolh; akin to OHG.
   selah, Dan. sael, Sw. sj[aum]l, Icel. selr.] (Zool.)
   Any aquatic carnivorous mammal of the families Phocidae and
   Otariidae.
   [1913 Webster]

   Note: Seals inhabit seacoasts, and are found principally in
         the higher latitudes of both hemispheres. There are
         numerous species, bearing such popular names as sea
         lion, sea leopard, sea bear, or ursine seal,
         fur seal, and sea elephant. The bearded seal
         (Erignathus barbatus), the hooded seal (Cystophora
         cristata), and the ringed seal (Phoca foetida), are
         northern species. See also Eared seal, Harp seal,
         Monk seal, and Fur seal, under Eared, Harp,
         Monk, and Fur. Seals are much hunted for their
         skins and fur, and also for their oil, which in some
         species is very abundant.
         [1913 Webster]

   Harbor seal (Zool.), the common seal (Phoca vitulina). It
      inhabits both the North Atlantic and the North Pacific
      Ocean, and often ascends rivers; -- called also marbled
      seal, native seal, river seal, bay seal, land
      seal, sea calf, sea cat, sea dog, dotard,
      ranger, selchie, tangfish.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sea dog
    n 1: a man who serves as a sailor [syn: mariner, seaman,
         tar, Jack-tar, Jack, old salt, seafarer, gob,
         sea dog]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "sea dog":
   AB, Ancient Mariner, Argonaut, Dylan, Flying Dutchman, Neptune, OD,
   Poseidon, Varuna, able seaman, able-bodied seaman, bluejacket,
   buccaneer, corsair, deep-sea man, fair-weather sailor, fisherman,
   freebooter, hearty, jack, jack afloat, jack-tar, jacky, limey,
   lobsterman, mariner, matelot, navigator, picaroon, pirate,
   privateer, rover, sailor, salt, sea robber, sea rover, sea wolf,
   seafarer, seafaring man, seaman, shipman, tar, viking, water dog,
   whaler, windjammer, windsailor