And I think you are right, Hendrix. In fact I know you are, because of the flight shots.
Just for the record I didn't see it in flight or flapping its wings - in which case alarm bells would certainly have rung - and juv Red-crested Pochard has an all-dark bill like this bird, though a different shape.
Also as Mullarney et al (Collins Bird Guide) put it juv/female R-c Pochard is "like a large pale Common [Black] Scoter". And I did say to another observer the bill shape and long, 'stiff' tail looked odd.
Oh well, it was only a first for HK which I misidentified!
Mike Turnbull.
PS Mullarney et al also suggest separating them on the basis of habitat!