UK and France had partly driven the directive.
However I doubt any UK regulation replacing the UK implementation of the EU directive is being even considered - Mother Nature will hope I am all wrong.
The issue here is straightforward. All EU regulations / legal recommendations etc that must be maintained so that the UK shows equivalents levels for future trade arrangements have simply been drafted by copy-pasting those existing EU frameworks.
But this isn't necessary for the waters directive.
On a similar sarcastic note, fishermen don't have to worry. Now that Great Britain is becoming Little US, fishermen will still have reservoirs.