The Collaborative International Dictionary of English v.0.48:

Tank ship \Tank ship\, Tank vessel \Tank vessel\ . (Naut.)
   A vessel fitted with tanks for the carrying of oil or other
   liquid in bulk; -- called also tanker. A tank ship of very
   large capacity is called a supertanker.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

tanker
    n 1: a cargo ship designed to carry crude oil in bulk [syn: oil
         tanker, oiler, tanker, tank ship]
    2: a soldier who drives a tank [syn: tanker, tank driver]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "tanker":
   argosy, ark, bathyscaphe, caravel, cargo ship, coast guard cutter,
   coaster, collier, derelict, dredge, fishing boat, freighter,
   hydrofoil, icebreaker, lash, lightship, liner, merchant ship,
   merchantman, oiler, packet, packet boat, paddle steamer,
   picket ship, refrigeration ship, revenue cutter, rotor, rotor ship,
   screw steamer, self-propelled barge, ship, side-wheeler, slaver,
   spar-decker, steam schooner, steam yacht, steamer, stern-wheeler,
   storeship, tank corpsman, tank crewman, tender, tramp steamer,
   transport, trawler, turbine, weather ship, whaler