Water Rats are rodents, but not part of the rattus genus, they are in their own designated genus. There are 4 species in the genus, 3 of them native to New Guinea and the fourth is this little guy, Hydromys chrysogaster.
Proficient swimmers, they are able to swim as effectively submerged as on the surface.
They are usually solitary creature, coming together to form pairs for mating.
A name change to Rakali, the traditional name, has been proposed and pushed to get their association with rats out of people's thinking.