My policy (Haven Knox-Johnson through Bay Marine Consultants Ltd) stipulates:
"Inland and Coastal waters of the United KIngdom, Eire and Continental waters between River Elbe and La Rochelle plus Inland Continental waters of Europe not South of 46 degrees North not North of 55 degrees North and not East of 10 degrees East".
I wanted to cross from Poole (south coast of England) to Braye (Alderney; CI)
Being a lawyer (sorry!) I was concerned that the policy covered only the "coastal waters" of England and of the Continent and that I would, therefore, be uninsured for the middle part of the crossing (see above threads).
There is, in fact, no universal definition of 'coastal waters' but 'Territorial Waters' are internationally accepted (for GB at least) as being 12 miles off shore (after allowing for bays, close lying islands etc to be included as part of the mainland). This would leave me uninsured for the middle part (total crossing 70nm).
I phoned my brokers who quickly apprciated the point and contacted the insurers to clarify it. The brokers promptly phoned back to advise that I would be insured for the whole crossing whenever I chose to go (with no conditions about two boats or prior notification etc) and have subsequently written to me confirming this.
I suspect that interpretation may well vary from insurer to insurer but I recommend doing as above to avoid problems.