Nutria???

[runamuk]

So where are these things found in high concentrations? Sounds like a good critter to practice skinning/tanning with. That was their real original purpose anyway right?

some of the densly populated lakes have had issues in recent years they are vicious little things and attack pets and people at times...look at lakes in king county and pierce county, I have seen them in thurston and grays harbor counties as well...

[PatoLoco]

Since you usually see them swimming and can only see the head- One good way to tell a nutria from something native (muskrat, beaver, otter) is a nutria's nose/nostrils are just holes in its snout whereas the others have distinct fur-free noses like a dog or cats.
Binoculars help. And Google image searches

[Hermit]

I have tons of beaver in the pond in my front yard. As soon as you spook a beaver it will slap the water with it's tail..... HARD.  Nutria just turn away and swim......I wouldn't eat one either. NEVER been that hungry.

[JJB11B]

Beaver or an Otter