There is underwater viewing of beavers at the Cleveland Zoo, Minnesota Zoo, Oregon Zoo, Northwest Trek Wildlife Park, Arizona-Sonora Desert Museum and I believe Knoxville Zoo and Omaha's Henry Doorly Zoo, amongst others..
Cleveland is my 'home' zoo and I can say that the Beaver exhibit while being very nice in theory, isn't so great. The animal rarely (I've never seen it) comes out of the den area. (On a side note, GREAT SHOT!) The den glass is always way too dirty to see through also, which is fine I mean it is an animal! But I wrote to say that Toronto Zoo also has underwater viewing of Beavers and was quite nice. A nice large glass window, and an above ground area where you can look down onto the animals.
I was also going to say, how I long to see a beaver out of its den! Or even one that moved slightly in its den. Why do I have a love for nocturnal animals?