Saw this guy passing by in an area where beavers are apparently resident (having been observed by others, plus visible traces of tree felling activity).
The one thing that makes me a bit unsure of the ID is that the lower back is visible above the water line. I've heard that in beavers, typically only the head and shoulders show, while Coypu and other swimming rodents swim with their back well visible. Also, Coypu are resident in a place not very far away from where this animal was seen (albeit a different habitat).
The critter was at least as big as a cat, so Muskrat should be out of the question.