Let's be specific... if it is kind of football shaped (humped back in water), with a naked tail it's either a muskrat or a beaver. If about 2 feet long (including naked tail that is rounded) it's a muskrat. If a broad flat naked tail, it's a beaver.
If it is long and skinny with a furred tail, it's either a mink or an otter.
If it's app. 2 feet long (or less), it's likely a mink.
If it's more than 2 feet long, even up to 4 feet, it's an otter. Otters are rare, but they exist.
Based upon your description of "long sleek," I would bet on a mink.
See <a href=https://brtthome.files.wordpress.com/2015/12/p1720753-1-2.jpg>this picture</a>.