And who is going to enforce that?
Talking about UK waters is about as nonsensical as Trump's wall. It's a sea. It doesn't belong to any sovereign nation. Perhaps some people who are old enough to remember the Cod Wars of the early 70s were young enough then to remember that territorial waters which had been drawn 12 miles from the shore then miraculously became 50 miles because a minister passed a statutory instrument. I don't think there was a walled or fenced border guarded by snipers. As I recall we had a dedicated frigate policing hundreds of miles of open sea.
What about the fish? How do we know they are British fish as opposed to illegal migrants? Di they need passports to enter British waters? Or are we happy to let them all in without proof of means of self-support..
There are enforceable laws that apply to the sea, just as there are to land. In the sea's case, by UN Convention. Territorial waters and EEZ's exist, even if they cannot be seen by the naked eye.