I'd say you could see water voles any time of day or night, but I just popped down to the field now - 8pm, coming up to dusk - and it was very busy indeed. Three sightings in the fifteen minutes I was there.
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?