water rat

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various semiaquatic rodents, especially Neofiber alleni of Florida and southern Georgia, that resemble the muskrat but are smaller and have a round tail.
• noun Slang A petty thief or ruffian who frequents waterfronts.

from The Century Dictionary.

• noun One of several different rodents, of aquatic habits, belonging to the family Muridæ.

from the GNU version of the Collaborative International Dictionary of English.

• The water vole. See under vole.
• The muskrat.
• The beaver rat. See under beaver.
• A thief on the water; a pirate.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US Any of various rodents from Florida and southern Georgia, especially Neofiber alleni; the muskrat.
• noun informal The Eurasian water vole, Arvicola amphibius (or Arvicola terrestris).
• noun US, slang A petty thief or waterfront ruffian.
• noun slang A person fond of water sports.
• noun Australia A member of the water police.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various amphibious rats
• noun a person who enjoys being in or on the water
• noun common large Eurasian vole