Definition of water rat:
(n) :
(US) Any of various aquatic or semi-aquatic rodents from Florida and southern Georgia, especially Neofiber alleni; the muskrat.
(n) :
(Australia) A water mouse, especially, the rakali, Hydromys chrysogaster.
(n) :
(informal) A European water vole (Arvicola amphibius, formerly Arvicola terrestris).
(n) :
(US, slang) A petty thief or waterfront ruffian.
(n) :
(slang) A person fond of water sports.
(n) :
(slang, Australia) A member of the water police.