I'm not an expert but I think from the remnants of brown on the down around its neck that this is one of this year's cygnets (or possibly from late last year?) already transformed into a juvenile mute swan. It has just had its head dipped in the water and you can see droplets falling from its bill.
This was taken in Calne just a few minutes walk from my cottage, on the short stretch of the river where it doubles as the old canal, near to the Town Hall. Of course, being Calne, there's also an old beer bottle floating past (easily visible in big).