On the river Exe in Exeter I saw this bird diving into the water. I had a look in the tree and couldn't see a flash of blue suggesting it was a Kingfisher (perhaps a juvenile?). The bird dived a few times.
I cannot think of anything else that it could be - it could well be a juvenile but I'm struggling to tell from the video
Cin J
I've played around with freeze-frames and tried to enhance some stills however i can make out very little other than some white flashes, which would correspond to the white on a Kingfisher's neck, but from the behaviour alone i would say definitely Kingfisher.
Certainly a kingfisher as others have said. No way of ageing it from that though.