water rat

any of various rodents having aquatic habits.

the muskrat, Ondatra zibethica.

(in Australia and New Guinea) any of the aquatic rats of the subfamily Hydromyinae, especially of the genus Hydromys.

Slang. a vagrant or thief who frequents a waterfront.

Origin of water rat

First recorded in 1545–55

The surmulot, in some of its habits, resembles the water-rat.

Buffon's Natural History. Volume VI (of 10) |Georges Louis Leclerc de Buffon
The chase did not always afford them sufficient food, and at times they were forced to subsist on the water-rat.

A Manual of the Antiquity of Man |J. P. MacLean
The water-rat was as well as could be expected, but did not quite like the new aspect of affairs.

Mr. Sponge's Sporting Tour |R. S. Surtees
Carefully looking over that side again, the moorhen who had been out rushed back; the water-rat was gone.

Nature Near London |Richard Jefferies

The water-rat dives with a tremendous splash as he sees the big monster glide by his sedge home.

Pony Tracks |Frederic Remington

water rat

any of several small amphibious rodents, esp the water vole or the muskrat

any of various amphibious rats of the subfamily Hydromyinae, of New Guinea, the Philippines, and Australia

informal a person who is very fond of water sports