What might look like a beaver could be a muskrat, and that muskrat you saw might be a beaver. These 2 animals can look quite similar; they also have some habits that make it easy to confuse them. They are both aquatic animals that like to swim; they have a similar look. Both build dens near the water, but beavers will build dams and engineer the environment around them, while muskrats keep it simpler. The homes built by muskrats are much smaller and simpler. The tails are quite different; muskrats have a long, thin tail, and beavers are known for their short, flat, and wide tails. Finally, beavers are much larger than muskrats, but watching both swim, it can still be tricky to tell them apart, so just look at the tails, and it should be obvious.
