Cormorant on the Kennet

On Friday afternoon I got my first reasonably close-up view of a cormorant.  I was hurrying along the towpath beside Reading prison (yes, I know its actually just a remand centre now) when a large black bird popped up from under the water only about 5 metres from me.  I stopped, leaned against the railings and  watched it while it dived and resurfaced several times, apparently without catching anything.  Then it flew off eastwards along the river. 

As it was rather small for a cormorant and had a tuft of feathers on the back of its head, it occurred to me that it might be a shag.  However, the latter tend not to come inland in the way that cormorants do and, also, their tufts are on the top of their heads, not at the back.