We see muskrats swimming along our shoreline at least once a week. They're very skittish and once you spook them into the water, they don't pop back up within visible range. I think they're relatively uncommon around here due to most properties installing anti-erosion structures along their shorelines--a seawall or riprap with metal mesh. Also, we still have a healthy amount of predators, eagles, hawks, foxes, raccoons, pollutants and speeding cars.
They were common in the area where I grew up, but I only ever saw one (despite being a very outdoorsy kid). It was being attacked by a mink, and they were both too busy with the task to take notice of me.
Sad to see the clear decline in muskrat numbers.