I suspect
Q is correct as to the gist of the phrase, however; a couple of pedantically extraneous observations:
Dictionary Unabridged (with a reference to
Q's venerable O.E.D.) has this to say:
noun 1. the sea or ocean beyond the three-mile limit or territorial waters of a country.
2. Usually, high seas. a. the open, unenclosed waters of any sea or ocean; common highway.
b. Law. the area within which transactions are subject to court of admiralty jurisdiction.
Origin: bef.
1100; ME;
OE h�ah-sǣ]
Additionally. some quite old legal references differentiate between 'high seas' as opposed to the sea bed. Legally, property rights and ownership could have been granted for the shore line at low tide, (seen here:
http://www.mass.gov/czm/shorelinepublicaccess. htm) but no implied ownership of the sea (water) covering the same land at high tide. Since, in many parts of the world, the difference between low tide/high tide boundaries could be a large distance, the generally accepted distance of 3 miles (usually
nautical miles) became applicable (since extended to 12 miles or further by most nations) for settling
national legal boundaries as well... lending the definition further credence.