I love the shape they make when they swim. As far as I'm concerned, there's no finer sight in English/Welsh/Scottish* nature than a water vole cruising up a river in the evening light to the sound of birdsong.
This blog charts the fortunes of water voles in and around the Whitchurch area, North Shropshire. Water voles are one of the UK's most threatened mammals, extinct in many counties, and so it's vital they receive as much monitoring and protection as there is going. Here in Whitchurch we're lucky enough to have them right in the middle of town - how cool is that?