Thames Admiral,
CAP168 applies only to licensed aerodromes and is not the definitive guide to seaplane ops or landings in the UK.. Since flying training no longer requires a licensed aerodrome, that's why there are no longer any licensed water aerodromes in the UK. I believe Loch Lomond Seaplanes operate their Public Transport flights using a CAA Exemption to land at unlicensed sites.
Different rules apply in Scotland. Land at a private lake in England or Wales without permission and you could be guilty of trespass.
Hamish Mitchell, David West or Neil Gregory are the Scottish experts on seaplaning...