'Water rat' definitions:

Definition of 'water rat'

noun

A person who enjoys being in or on the water [syn: water dog, water rat]

noun

Common large Eurasian vole [syn: water vole, water rat, Arvicola amphibius]

noun

Any of various amphibious rats

From: GCIDE

Water rat \Wa"ter rat`\
1. (Zool.) (a) The water vole. See under Vole. (b) The muskrat. (c) The beaver rat. See under Beaver. [1913 Webster]
2. A thief on the water; a pirate. [1913 Webster]

From: GCIDE

Muskrat \Musk"rat`\, n.
1. (Zool.) A North American aquatic fur-bearing rodent (Ondatra zibethica formerly Fiber zibethicus). It resembles a rat in color and having a long scaly tail, but the tail is compressed, the hind feet are webbed, and the ears are concealed in the fur. It has scent glands which secrete a substance having a strong odor of musk. Called also musquash, musk beaver, ondatra, and sometimes water rat. [1913 Webster +PJC]
2. (Zool.) The musk shrew. [1913 Webster]
3. (Zool.) The desman. [1913 Webster]