water rat

noun6

1. •
   (US) Any of various aquatic or semi-aquatic rodents from Florida and southern Georgia, especially Neofiber alleni; the muskrat.

   • Otters are even harder to spot; the coypu, a North American water rat, is more commonly seen. — 2004
2. •
   (Australia) A water mouse, especially, the rakali, Hydromys chrysogaster.

   • "There [...] you will find the water-hole that is sacred to Biggoon, the giant water rat." — 1943
3. •
   (informal) A European water vole (Arvicola amphibius, formerly Arvicola terrestris).
4. •
   (slang) A person fond of water sports.
5. •
   (US, slang) A petty thief or waterfront ruffian.
6. •
   (Australia, slang) A member of the water police.